a and b can do a piece of work in 7 days . with the help of c they finish the work in 4 days . c alone can do that piece of work in ?
"c = 1 / 4 â € “ 1 / 7 = 3 / 28 = > 28 / 3 = 9 1 / 3 days answer : b"
inverse(subtract(4, divide(4, 7)))