A complete 710 of a work in 15 days, then he completed the remaining work with the help of B in 4 days. In how many day A and B can complete entire work together?

Answer: Option C

$$\frac{7}{10}$$ part of work has been completed by A in 15 days. Then,
Rest work = 1 - $$\frac{7}{10}$$ = $$\frac{3}{10}$$ part
Given, That $$\frac{3}{10}$$ part of the work is completed by A and B together in 4 days. Means,
(A + B) completed the $$\frac{3}{10}$$ of work in 4 days
So, (A + B)'s 1 day's work = $$\frac{3}{10\times 4}$$  = $$\frac{3}{40}$$
Hence,
(A + B) can complete the work in $$\frac{40}{3}$$ = $$13\frac{1}{3}$$ days