A can do a piece of work in 15 days, which B alone can finish in 12 days. Both together work for 4 days and then A leaves off. How many days will B take to finish the remaining work ?    

A's one day work $=\frac{1}{15}$

B's one day work $=\frac{1}{12}$

A & B together 4 days work = $4(\frac{1}{15}+\frac{1}{12}) = 4\times \frac{9}{60}=\frac{3}{5}$

Remaining work $(1-\frac{3}{5}=\frac{2}{5})$ can finish by B in 

1 work in 12 days

$\frac{2}{5}th$ work in $=\frac{2}{5}\times 12$ days = 4.8 days