So we know that both of them can mow the lawn in 71 minutes, and we also know that Billy can mow the lawn by himself 30 minutes faster, so we say 71/(t - 30) + 71/(t) = 1. Now multiply both sides by t(t - 30) to get 142t -2130 = t^2 - 30t. Simplify to get t^2 - 142t + 2130 = 0. Use the quadratic formula now to get t

142 +/- sqrt(142^2 -8520) over 2. Simplified would be t = 125 minutes Hope this helps.