Rituraj is 66 2 3 efficient than Sameer and he complete a piece of work with the help of Raj in 10 d...........

Rituraj is 66 2/3 % ???????? efficient than Sameer and he complete a piece of work with the help of Raj in 10 days. If all three worked together and take 7 1/2 ???????? to complete the same work, then find in how many days Raj alone will complete the same work?