Twin brothers, Billy and Bobby, can mow their grandparent's lawn together in 53 minutes. Billy could mow the lawn by himself in 20 minutes less time that it would take Bobby. How long would it take Bobby to mow the lawn by himself?

5353 minutes = 89.22 hours

Maybe the figures should be 53 and 20?

Let's let Billy = A

The formula to use for work problems is

Time = (A*B) / (A+B)

53 = ((A + 20) * A) / (A + 20 + A))

53 = (A^2 + 20A) / (2A + 20)

A^2 + 20A - 106A - 1060

A^2 - 86A - 1060

A = 96.935

Bobby Mows in 116.935 minutes