a cistern is filled by pipe a in 20 hours and the full cistern can be leaked out by an exhaust pipe b in 25 hours . if both the pipes are opened , in what time the cistern is full ?
"time taken to full the cistern = ( 1 / 20 - 1 / 25 ) hrs = 1 / 100 = 100 hrs answer : e"
divide(const_1, subtract(divide(const_1, 20), divide(const_1, 25)))