Two mowers sharing one lawn

One gardener mows a lawn alone in $60$ minutes; a slower one needs $75$ minutes. Working together on the same lawn, how many minutes do they take?

$100/3\approx33.3$ minutes. Their rates add: $\tfrac1{60}+\tfrac1{75}=\tfrac{9}{300}=\tfrac{3}{100}$ lawn per minute, so one lawn takes $\tfrac{100}{3}$ minutes.