a and b together can do a work in 9 days . if a alone can do it in 18 days . in how many days can b alone do it ?
"1 / 9 – 1 / 18 = 1 / 18 = > 18 answer : c"
add(inverse(subtract(divide(const_1, 9), divide(const_1, 18))), divide(const_2, add(const_2, const_3)))