a can do a job in 15 days and b in 20 days . if they work on it together for 3 days , then the fraction of the work that is left is ?
a ' s 1 day work = 1 / 15 b ' s 1 day work = 1 / 20 a + b 1 day work = 1 / 15 + 1 / 20 = 7 / 60 a + b 3 days work = 7 / 60 * 3 = 7 / 20 remaining work = 1 - 7 / 20 = 13 / 20 answer is c
subtract(const_1, multiply(3, add(divide(const_1, 15), divide(const_1, 20))))