a and b together can do a work in 5 days . if a alone can do it in 15 days . in how many days can b alone do it ?
"1 / 5 – 1 / 15 = 2 / 15 = > 7.5 answer : c"
add(inverse(subtract(divide(const_1, 5), divide(const_1, 15))), divide(const_2, add(const_2, const_3)))