S can do a piece of work in 80 days and T in 60 days. With help of U, they finish the work in 20 days. In how many days U alone can do the same work?

S can do a piece of work in 80 days and T in 60 days. With help of U,

1.	S can do a piece of work in 80 days and T in 60 days. With help of U, they finish the work in 20 days. In how many days U alone can do the same work?
A.	40
B.	60
C.	48
D.	80
Answer» D. 80