A can complete 9/10 part of a work in 31 days. After that, with the help of B he can complete the remaining work in 4 days. In how many days will A and B together can complete that work ?

Remaining work = 1 - 9/10 = 1/10

=> A & B together completes 1/10 of work in 4 days

=> 1 work can completed in ------ ? days

Let it be x days

=>   $\frac{4}{\frac{1}{10}}=\frac{x}{1}$

=> x = 40 days.

Hence, A & B together can complete the work in 40 days.