a can do a job in 15 days and b can do it in 30 days . a and b working together will finish twice the amount of work in - - - - - - - days ?
"c 1 / 15 + 1 / 30 = 1 / 10 10 * 2 = 20 days"
add(divide(const_1, 15), divide(const_1, 30))