Problem Solving

Please explain me in the fast way to solve this problem. I always have problem with work problem. What is the concept to solve this kind of problem?

Lindsay can paint 1/x of a certain room in 20 minutes. What fraction of the same room can Joseph paint in 20 minutes if the two of them can paint the room in an hour, working together at their respective rates?

a. 1/3x

b. 3x/x – 3

c. x – 3/3x

d. x/x – 3

e. x – 3/x

L=20x
1/L + 1/J = 1/60
therefore 1/J = 1/60 - 1/20x or J = 60x/(x-3) and threfore in 20 minutes it should be 3x/x-3.

Simplest way to solve is this:

L does 1/x work in 20 min so she will do 3/x in 60 min

3/x + J's work in 60 = 1

J's work in 60 min = 1-(3/x)

J's work in 20 mins = (1/3) * (1-[3/x]) = (x-3)/3x

I agree with netigen. What's the OA?

I agree with netigen. What's the OA?

Please post the answer to the question.

thanks

ans -C