carpeD wrote:

Machine A can complete a certain job in x hours. Machine B can complete the same job in y hours. If A and B work together at their respective rates to complete the job, which of the following represents the fraction of the job that B will not have to complete because of A's help?

(x - y)/(x + y)

x/(y – x)

(x + y)/xy

y/(x – y)

y/(x + y)

Just plug in some values. X=2hrs Y=4hrs.

So rates are 1/2 and 1/4. Combined they are 3/4. So it takes 4/3 hrs to complete the job working together. Or 1hr 20min.

Subtract 4hrs from 1hr 20 min. Difference is 2 2/3 or 2 hrs 40min.

So calculate how much work Y does in 2 2/3 hrs (8/3hrs). 1/4* 8/3 = 2/3 of the job.

So look for the choice that gives you this value.

A. 2-4/4+2 = -2/3 no.

try em all.

E: 4/2+4 = 2/3

This method is a bit slower, and its not completely full proof. Ex/ if you chose x=4 and y=2. Then A and E work. In this situation ud have to go back and calculate the difference of the time, then calculate how much work Y did. But in any case if u didn't know wut to do, u at least only have 2 choices left.