Question

Twin brothers, Billy and Bobby, can mow their grandparent's lawn together in 109 minutes. Billy could mow the lawn by himself in 20 minutes less time than it would take Bobby. How long would it take Bobby to mow the lawn?

Answer 1

Let X = time for Bobby to mow the lawn

Then X-20 = time for Billy to mow the lawn.

Together they mow the lawn in 109 minutes. So, that provides us with:

X + (X-20) = 109

Simplify and solve for X (which is bobby's time).