working alone , printers x , y , and z can do a certain printing job , consisting of a large number of pages , in 15 , 12 , and 18 hours , respectively . what is the ratio of the time it takes printer x to do the job , working alone at its rate , to the time it takes printers y and z to do the job , working together at their individual rates ?
"the time it takes printer x is 15 hours . the combined rate of y and z is 1 / 12 + 1 / 18 = 5 / 36 the time it takes y and z is 36 / 5 the ratio of times is 15 / ( 36 / 5 ) = 5 * 15 / 36 = 25 / 12 the answer is d ."
divide(15, divide(const_1, add(divide(const_1, 12), divide(const_1, 18))))