It takes Bobby, Carl and David 15 minutes to weed a garden. Bobby can weed the garden by himself in 50 minutes, and Carl can weed it in 30 minutes. How long would it take David to weed the garden if he worked alone?

Solution:

it takes David x mins to weed the garden if he worked alone

So

1/x+1/30+1/50 = 1/15

x = 75

So it takes David 75 minutes to weed the garden if he worked alone