a , b and c can do a piece of work in 7 days , 21 days and 28 days respectively . how long will they taken , if all the three work together ?
"1 / 7 + 1 / 21 + 1 / 28 = 19 / 84 = > 84 / 19 = > 4 8 / 19 days answer : b"
inverse(add(inverse(28), add(inverse(7), inverse(21))))