two pipes can fill a tank in 18 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 / 18 + 1 / 15 - 1 / 45 = ( 5 + 6 - 2 ) / 90 = 9 / 90 = 1 / 10 so , the tank becomes full in 10 minutes . answer : d :"
subtract(add(divide(const_1, 18), divide(const_1, 15)), divide(const_1, 45))