A is thrice as efficient as B. A started working and after 4 days he was replaced by B. B then worked for 15 days and left. If A and B together finished 75% of the total work, in how many days B alone can finish the whole work ?

Rate of doing work by A and B is $$\frac{1}{A} $$ and $$\frac{1}{B}$$
$$\frac{1}{A} $$ = $$\frac{3}{B}$$
In 4 days A would have completed $$\frac{4}{A}$$ or $$\frac{12}{B}$$ amount of work
In 15 days B would have completed $$\frac{15}{B}$$ amount of work
Since total work done was 75%.
$$\frac{12}{B} + \frac{15}{B} = \frac{3}{4}$$
$$\frac{27}{B} = \frac{3}{4}$$
$$\frac{1}{B} = \frac{1}{36}$$
B alone would take 36 days to complete the work.
Option D is the correct answer.