A completes $\frac{7}{20}$ of a work in 10 days. Then he completes the remaining work with the help of B in 5 days. The time required for A and B together to complete the entire work is :

$\frac{100}{13}$ days

A's 10 days work = 7/20

A's 1 day work = 7/200

Remaining work = 1 - 7/20 = 13/20

A and B work together for 5 days to complete 13/20 of the work

A and B one day work = (13/20)/5 = 13/100

Total time taken by A and B together to complete the work = 100/13

The correct answer is Option (2) → $\frac{100}{13}$ days