Beat The GMAT a GMAT & MBA community

Rate problems that do not give a specific amount for the job can best be solved by picking a number for the job so you don't have to work with fraction. In this case you can pick 36 becuase it divides easily into both 9 and 18.

The rate formula is D=R(t) and you often have to use this formula multiple times for different scenarios within a problem.

The first information we get is that M can do the job in 18 minutes that means that 18(M)=36 - thus the rate of M=2.

Next we get that N can do the job in 9 minutes that means 9(N)=36 - the rate of N=4

Then it asks if you are using the same number of each machine and you finish in one minute how many of each machine.

the task stays at 36 because it hasn't changed and the time we are looking for is 1. when machines work together you have to add their rates within the rate formula but here we have some number of each machine thus the combined rate is going to be the sum of that number times each rate.

This gives you the formula: (x(2)+x(4))(1)=36 - which can be solved to 6x=36 thus x=6 and there are 6 of each type of machine, thus the answer is C.

_________________
Becky
Master GMAT Instructor
The Princeton Review
Irvine, CA

Some good solutions posted already; we can also use the basic work formula for two workers:

CT = combined time
A = time for worker 1 to do job on its own
B = time for worker 2 to do job on its own

CT = (A*B)/(A+B)

In this case:

CT = 9*18/(9+18) = 162/27 = 6

Since it takes 6 minutes for 1 of each machine to complete the job, we need 6 of each machine to complete the job in 1 minute: choose (C)!

_________________
Stuart Kovinsky, B.A. LL.B.
Kaplan Test Prep & Admissions
Toronto Office
1-800-KAP-TEST
www.kaptest.com

Learn more about me