sum of 1-100

Gauss yo, 50 sets of 101, I learned that in second grade. Or just average 1 and 100, to get 50.5 x 100. 10,000 is a pretty awful guess since that would mean 100 x 100, when you know that's not possible.

Are you serious?

You're supposed to recognize that 1+ 100 is 101, 2+99 is 101, 3+98 is 101, etc.

101*50 = 5050

this was a really common SAT question way back, bruh

find the sum of 1-99 first, leaving out the 100:

1+99 = 100 2+98 = 100 3+97 = 100

and so on...

Think about it this way, you have 50 pairs that add up to 101...

100+1 99+2 98+3 97+4 96+5 etc.

50*101 = 5050

In my book, brainteasers and absurd mental math problems do not prove anything. Unfortunately, some firms and people enjoy giving them and therefore you should attempt to gain exposure to as many as possible. This way, in the likelihood that your interviewer is a dick, you can pretend you have not heard it and talk your way through it.

You can be sure they're not going to be calling you back.

1+2+3+ ... + 98 + 99 + 100 = 50*(101) = 5050.

I think this is taught to 11-13 year olds these days.

You're not the only one dude. I forgot about this until I started prepping for the GMAT.

I think u should at least recall 1 to n = [n * (n+1)]/2 and 1 to n^2 = [n * (n+1) * (2n +1)]/6 or how to split the pairs of numbers.

To be honest, for phone interviews I build back in my young days a simple Excel file with the most common questions. So sum 1 to n, 1 to n^2, 3rd roots, powers, permutations, combinations, etc. Didn't want to risk blacking out or something and I'm a lazy bitch. Seriously, if it's a phone interview: cheat! ;)

This is much simpler than you think.

The average of 1-100 is 101 / 2 or 55.5. Since the average is the sum of a set of figures divided by the number of figures in that set, you can get the sum by multiplying the average by the number of figures in the set. In this case there are 100 figures, the average is highest + lowest / 2 or (1+100)/2=55.5; so the sum is average x (number of figures) or 55.5 x 100 = 5,550.

101/2=50.5

50.5*100=5050

But yes, Marcus has the intuition right.

And brainteasers? Bro, wtf do you think a DCF is if you don't know some basic sums and series?

Well now i know...thanks to mikes super short show.

bush league bro...

http://www.ibankingfaq.com/category/interviewing-brainteasers/