a can do a job in 12 days and b can do it in 20 days . a and b working together will finish twice the amount of work in - - - - - - - days ?
"a 1 / 12 + 1 / 20 = 2 / 15 15 / 2 * 2 = 15 days"
add(divide(const_1, 12), divide(const_1, 20))