Consulting Interview Math

It's all math you should be able to do in your head or with a few quick scribbles. If you're doing long division you've screwed something up. They won't be asking you to do integral calculus or anything.

I'd add that you should be carefully writing out the major steps in your calculations so that you can keep track of what you are doing.

Yes, the math is arithmetic... but you are doing it under timed conditions in front of an interviewer for a job. You will be nervous and prone to make mistakes.

Additionally, it is easier to keep track of what you are doing if you are writing out the steps as you do them.

Finally, it is MUCH easier to explain you calculations if you are writing them out.

Make sure to speak slowly and clearly. Be methodical and take your time.

my advice would be to get quick at doing percents. Specifically, be able to get the multiples of 10 and 5 down...for example: What's 65% of 21,000?

You should be able to do that in your head.

Alright well the next question is: How fast do you have to be able to do these things? Are you allowed to actually write your calculations down (I assume so, according to vyger's comment), or do you need to be able to rattle them off in your head? Or are you just saying you should be able to do it in your head in order to move things along quickly?

"Okay, so we need 65% of 21,000. Well, let's round 65% to 2/3rds - it's within a couple percent. Two thirds of 21,000 is 14,000."

The expectation is that you can talk fluently through math like that. The problem comes when you mess things up, or when you do something like this:

"Okay, so we need 65% of 21,000..." [interviewee writes furiously for a silent minute] "Okay, 13,650."

^^^ that works (the 2/3rds part)

BUT, you can also impress interviewers by doing these calculations quickly and painlessly (if you are comfortable with this). I think of it as 60% of 21,000 is 12,600, and 5% of 21,000 is 1,050, add those together and you get 13,650. You really wouldn't need to write anything down other than your final figure, and that shouldn't take you more than 5-10 seconds.

Again, that is only if you are comfortable, the 2/3rds part works perfectly well and at the end of the day it is the big picture that matters with these companies. And to be honest I've lost some interviewers before with doing too much quick math...that didn't go too well.

I found that it helps to talk through the math out loud as you do it methodically on the paper. Helps engage the interviewer and also makes sure you stay on track and catch errors.

As in the rest of consulting, I think it's all about breaking the problem down in a way you can understand it, and in a way that you can explain well. So that works for the 2/3 approach and the 60% + 5% (how'd you do the 60%?) approach. To me, it seems most intuitive to break 65% into 50% + 10% + 5% (10,500 + 2100+ 2100/2). All easy to calculate. But yeah definitely talk through everything.