A and B togther can do a peice of work 12 days. A alone takes 21 days. The time taken by B alone is -

36 24 14 28

28

Efficiency of B = (A + B) - (A) = 7 - 4 = 3 Time taken by B alone = \(\frac{Total \; Work}{Efficiency}\)                                  = \(\frac{84}{3}\) = 28 days