A and B together can complete a piece of work in 8 days, B alone can complete that work in 12 days. B alone worked for four days. After that how long will A alone takes to complete the work ?

Answer: Option C

L.C.M. of total work = 24
One day work of A + B = $$\frac{24}{8}$$ = 3 unit/day
B's 1 day work = $$\frac{24}{12}$$   = 2 units/days
A's 1 day work = 3 - 2 = 1 units/days
4 days work of B = 4 × 2 = 8 units/days
Work left = 24 - 8 = 16 units
A will complete the remaining work in
$$\begin{array}{rl}& =\frac{\text{16 units}}{\text{1 unit/day}}\\ & =\text{16 days}\end{array}$$