a can do a piece of work in 20 days ; b can do the same in 30 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 / 20 + 10 / 30 + 10 / x = 1 x = 60 days answer : b"
divide(10, subtract(const_1, divide(add(10, 10), 20)))