A company bought for its 7 offices 2 computers of brand N

Hi, there. I'm happy to help with this.

This problem has to do with combinations. Here's the general idea: if you have a set of n elements, and you are going to choose r of them (r < n), then the number of combinations of size r one could choose from this total set of n is:

# of combinations = nCr = (n!)/[(r!)((n-r)!)]

where n! is the factorial symbol, which means the product of every integer from n down to 1. BTW, nCr is read "n choose r."

In this problem, let's consider first the three computers of brand M. How many ways can three computer be distributed to seven offices?

# of combinations = 7C3 = (7!)/[(3!)(4!)] = (7*6*5*4*3*2*1)/[3*2*1)(4*3*2*1)]

= (7*6*5)/(3*2*1) = (7*6*5)/(6) = 7*5 = 35

There are 35 different ways to distribute three computers to 7 offices. (The massive amount of cancelling that occurred there is very much typical of what happens in the nCr formula.)

One we have distributed those three M computers, we have to distribute 2 N computers to the remaining four offices. How many ways can two computer be distributed to four offices?

# of combinations = 4C2 = (4!)/[(2!)(2!)] = (4*3*2*1)/[(2*1)(2*1)] = (4*3)/(2*1) = 12/2 = 6

For each of the 35 configurations of distributing the M computers, we have 6 ways of distributing the N computers to the remaining offices. Thus, the total number of configurations is 35*6 = 210. Answer choice = B

Notice, we would get the same answer if we distributed the N computers first.

Number of ways to distribute 2 N computers to seven offices = 7C2 = (7!)/[(2!)(5!)] = (7*6)/(2*1) = 21

Number of ways to distribute the 3 M computers to the remaining five offices = 5C3 = (5!)/[(3!)(2!)] = (5*4)/(2*1) = 20/2 = 10

Product of ways = 21*10 = 210. Same answer.

Here's a blog article I wrote about this topic, with some practice questions.

https://magoosh.com/gmat/2012/gmat-permu ... binations/

Does this make sense? Please let me know if you any questions on what I've said.

Mike