GMAT Problem Solving : Rates – Work Time

Percy can mow 1/a of a lawn in half an hour. Mona and Percy working together at their respective rates can mow the entire lawn in a couple of hours. How many minutes will Mona take to mow the lawn?

 40

Correct Answer : Choice A.

Explanatory Answer

Percy can mow 1/a of the lawn in 30 minutes
So, Percy can mow the lawn completely in 30a minutes.

Therefore, in 1 minute, Percy will mow 1/30a of the lawn.

Percy and Mona, together, can mow the lawn in 120 minutes.
So, together they mow 1/120 of the lawn.

If Mona alone can mow the lawn in M minutes, then 1/30a + 1/M = 1/120

Or 1/M = 1/120 – 1/30a
1/M = (a – 4)/120a

Or M = 120a/(a – 4).

Alternative way to solve this question.

Assume a value for a. Say a = 5.
So, Percy mows 1/5th of the lawn in 30 minutes.
Percy can mow the entire lawn in 150 minutes.

Percy and Mona mow the lawn in 120 minutes

So, if Mona takes M minutes to mow the lawn, 1/150 + 1/M = 1/120
Or 1/M = 1/120  – 1/150 = 1/600
Or M = 600.

Substitute a = 5 in the answer choices and check which of the answer choices results in 600.
Choice A : M = 120a/(a – 4)
If a = 5, we get M = 120*5/(5 – 4) = 600.

So, choice A is the correct answer.