two pipes can fill a tank in 20 minutes and 15 minutes . an outlet pipe can empty the tank in 45 minutes . if all the pipes are opened when the tank is empty , then how many minutes will it take to fill the tank ?
"part of the filled by all the three pipes in one minute = 1 / 20 + 1 / 15 - 1 / 45 = ( 9 + 12 - 4 ) / 180 = 17 / 180 so , the tank becomes full in 5 minutes . answer : a"
subtract(add(divide(const_1, 20), divide(const_1, 15)), divide(const_1, 45))