a can do a piece of work in 40 days ; b can do the same in 40 days . a started alone but left the work after 10 days , then b worked at it for 10 days . c finished the remaining work in 10 days . c alone can do the whole work in ?
"10 / 40 + 10 / 40 + 10 / x = 1 x = 20 days answer : a"
divide(10, subtract(const_1, divide(add(10, 10), 40)))