Speed up Mental Math 5 times in 1 Week. It Works!

one of my biggest fears growing up was to be forced to do mental math in pressured situations I was really bad at negotiations that involved complicated numbers I always got beaten in all mental math games and eventually I failed the practice case a McKenzie PST practice badly and math was my biggest weakness I managed to get through the recruitment process by making the qualitative part compensate but once in McKenzie the mental math nightmare came to haunt me again literally every hour working in McKenzie involves mental math meeting with clients calling high-profile partners problem solving sessions there's simply no escape so over the years I've developed a system for myself of doing math so fast in my head one week of intensive practice and I became the fastest and calculating numbers on the team I even beat my own e M so what's the secret [Music] hi my name is Kim Tran a former McKinsey consultant and the founder of the management consulting prep platform now let's set up a theme here what's the difference between consulting math and the type of math that we all learn in school first there's a lot of it in a normal day a management consultant encounters many more calculations than most professions out there second most of the calculations in consulting need really fast answers even a really quick act of pulling out the calculator and inputting numbers is already considered too slow imagine you're interviewing a key client expert in getting some really intriguing figures you need to instantly do some mini analysis in your head and incorporate that into the best next question without any delay fortunately most of the time we don't need to be 100% precise up to a five percent margin for error is acceptable most case interviewers accept this and most of the questions in the PST have answer choices far enough away from each other this profoundly changes the way we approach most math calculations in consulting contexts many times that we can trade a tiny bit of accuracy for a lot more speed with my method you'll eventually be able to do calculations with just a two to three percent margin for error it's all about estimation and adjustment in the estimation phase we round the number into simple version we can easily work with and the adjustment phase we take the result and adjust up or down for the rounding we did earlier depending on the level of accuracy required we can spend more or less effort on this adjustment phase with casual calculations ballpark adjustment can give us a 5% margin just fine with tighter situations a few tricks will give us a much more precise answer with some trade often time of course in this video I'll give you a closer insight into how to do mental math with three of the most popular types of calculations and consulting multiplication division and percentages let's start the party with multiplication this type of calculation is the most simple and it's the foundation to learn many mental math techniques suppose we have a calculation of 1234 times 567 step 1 put away zeroes and round 1234 and 567 become 12 point 3 4 and 5 point 6 7 round them up or down to the nearest whole number and we're ready for step 2 Sept to do the simple multiplication then adjust it depends on the degree of upward or downward rounding you made in step 1 in general you would need an upward adjustment if a downward rounding has been made and in contrast a downward adjustment would be necessary if an upward rounding has been conducted in the end returned the number of zeros which has been put away in step 1 now you have your answer let's take a typical example 8710 times 590 for step one putting away zeros and rounding we will have a nice simplified version of eight point seven times five point nine taking out five zeros in total next step is to round eight point seven now becomes nine so it needs some downward adjustment later five point nine now become six so it needs a tiny bit of downward adjustment later as well step two multiplying and adjusting nine times six equals 54 it's important to adjust the rounding before putting in the zeros both of the above rounding needed downward adjustments so I would ballpark it to be a rather big adjustment 54 will be adjusted to 51 so the final answer is five million one hundred thousand a more precise version from the eight point seven we have examined two ends of the possible rounding they times 6 is 48 and 9 times 6 is 54 so eight point seven times six is somewhere in between but closer to 54 of course even more precisely eight point seven is 2/3 away from eight to nine so the final answer is somewhere 2/3 away from 48 to 54 as well 52 seems to be a good estimation and adjusts a little bit down for the 5.9 instead of 650 1.5 seems right putting the zeros in we have 5 million 150,000 and the correct answer is actually five million one hundred seventy three thousand seven hundred forty see it isn't difficult at all is it alright let's try another example disclaimer I'm using Excel to randomly generate these calculations 1662 times 6514 step one take out six zeroes and round we have a computation of one point six six times six point five because most people know one point six six times three equals five it's better to calculate one point six six times six step two one point six six times six equals ten adjust up lazy ballpark method would give us eleven a more precise method would say point five times one point six six equals roughly point eight so the adjusted one would be ten point eight putting in the zeros and we have ten million eight hundred thousand the correct answer is ten million eight hundred twenty-six thousand two hundred sixty-eight I know you're so close to mastering it let's do another example this time I want you to do it yourselves then compare yours with my method you have five seconds to tackle this one I'm still using Excel random function to generate these calculations ready go seventy-six thousand nine hundred twenty eight times eight thousand six hundred ninety one okay how was it here's my method step one take out six zeros in total and becomes seventy six times eight point seven then round we take seventy five times eight because most people know seventy five times two equals 150 seventy five times eight equals six hundred we will need some big adjustments later but this rounding makes the calculations so much faster step two seventy five times eight equals six hundred adjust up 0.7 of 75 is roughly fifty adjust up another time for the seventy six the total adjustment should be sixty we have six sixty with zeros we have six hundred sixty million the correct answer is six hundred sixty-eight million five hundred eighty one thousand two hundred forty eight that's how you do multiplication what about the other types of calculations now let's go for the reverse version of multiplication division believe me it's not hard at all suppose we need to calculate a b c d / e f g the general method is to find how much EF g needs to be multiplied to equal ABCD then we follow quite similar process of estimation and adjustment the subtle difference between a multiplication and division lies in returning zeros in the last step the number of zeros returned equals the number of zeros put away in the numerator minus the number of zeros taken out in the denominator okay let's look at an example 8,500 9 divided by 45 step one estimate will have a nice version of 85 divided by 45 taking out two zeros in the numerator only there's no need to do round as 85 and 45 I've already been whole numbers we then turn division into multiplication and find X 45 times x equals 85 45 times two equals 90 thus X is approximately two step to adjust as ninety is slightly higher than eighty five we need to adjust it down turn to into 1.9 then return to zeros you have taken out in step one we get the answer up 190 the correct answer is one eighty nine point zero eight nine starting to feel familiar let's try another example yourselves you have five seconds go ninety six thousand eight hundred forty divided by five hundred twenty-nine step1 estimate we will have a simplified version of ninety six point eight four divided by 52 point nine taking out three zeros in the numerator and one zero in the denominator which means that we will return to zeros in the last step technically we need a step of rounding here but in this case we can just skip it and make an overall adjustment later now how turn division into multiplication and find X 52 point nine times x equals roughly ninety six point eight we know 53 times two equals 106 thus X is a little bit lower than two step to adjust with the downward adjustment for two we have one point eight and with zeros it's 180 the correct answer is 183 0.06 that's how you do division do you still wonder about the other types of calculations last but not least I'm going to bring you to another popular type of calculation percentage its last but certainly not least with the computation a B percent of cdefg simply you just need to ignore percent turn it into multiplication of a b times c d e f g then take away two zeros sounds easy huh sometimes when you're lucky the percentage is already close to some easy figure like 33 percent 50 percent or 75 percent you can deploy even more shortcuts to cut the time and don't need to do the multiplication and divide 100 like above however the approach still goes through two stages estimation and adjustment let's look at an example seventy percent of fifteen thousand nine hundred forty we can see seventy percent is not so close to any special figure but it's very round already so we would apply the classic approach of calculating step one multiplied seventy times fifteen thousand nine hundred forty take out four zeros and rounding we have seven times sixteen that equals to one hundred twelve adjust it down just a tiny little bit we have one hundred eleven point mine returned four zeros and take out two zeros originally from the percentage sign and our answer is 11190 sometimes working with that many zeros can be complicated and you'll make mistakes so I have this simple trick to do a sanity check with the original number with percentage calculations the final answer has to be sizeable compared to the original number in this case 11000 is sizable compared to 15,000 if my calculation shows 110,000 my safety belt should definitely be ringing the correct answer is 11000 158 find it easy let's try another example yourselves three two one and go 32% of 650 1040 with this computation you're lucky enough to realize that 32% is very close to one-third you can still calculate using the multiplication method however it's quicker if you cling to this way let's try it out step one estimate one third of six hundred fifty one thousand forty is one third of six hundred thousand plus one third of fifty one thousand that's roughly two hundred thousand plus seventeen thousand which is two hundred seventeen thousand step two adjust as thirty-two percent is less than one two third which is thirty three point three percent it's obvious to apply a little bit more than one percent downward adjustment which is a little more than six thousand I would adjust it to about seven thousand my final guests would be two hundred ten thousand the correct answer is two hundred eight thousand three hundred thirty-two okay let's take a broad view again the key of this whole game is to be able to do adjustment quickly and with the high accuracy rate at first it seems hard but with practice you'll be amazed at how quickly your adjustment skills improve in real life we do these adjustments all the time think about the time you drive your car up the hill and still try to maintain the same speed how much more gas do you need to push is a complicated question of computation with several inputs of a what gear your car is in B how fast you're traveling see how steep the slope is D the wind the friction etc in real life nobody does those calculations we really all just do it by feel gathered from driving the car through various hills thousands of times computation using my method is very similar just try to do it every day and your adjustment skills will be amazing in no time now you know my way to excel a mental map maybe it's time for you to master it it may be hard in the beginning but soon you'll be astonished and excited by how you can handle such big number calculations in such a short time trust me because I did practice makes perfect and I know you can don't forget to share this video with your friends when you get better please tell us what you think and come in the section down below I'm also building a forum for you to practice and compare the results with your peers when it's ready the link will be in the description box there are many other small tips to be incorporated into this big method and it would be nice to learn together as a big community ad management consulting prep we believe everybody can excel at mental math are you a believer

Channel: MConsultingPrep Official Channel

Views: 220,441

Keywords: consulting, mental math, management consulting, consulting math, consultant

Id: _bktsTc4PLI

Length: 16min 10sec (970 seconds)

Published: Mon Sep 16 2019