a can do a job in 15 days and b in 20 days . if they work on it together for 4 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 4 days work = 7 / 60 * 4 = 7 / 15 remaining work = 1 - 7 / 15 = 8 / 15 answer is b"
subtract(const_1, multiply(4, add(divide(const_1, 15), divide(const_1, 20))))