A, B and C worked together for 5 days to build a wall and then B left the work. A and C together finished the remaining work in 5 days. In how many days can A and C working together finish building the whole wall if B alone can do it in 25 days?

$6\frac{1}{4}$

Let the total work = 1

Work done by A, B and C in five days = \(\frac{1}{5}\)

Remaining work = 1 - \(\frac{1}{5}\) = \(\frac{4}{5}\)

Now according to the question,

\(\frac{4}{5}\) work done by A, C in = 5 days

1 work done by A, C  in = 5 × \(\frac{5}{4}\) = \(\frac{25}{4}\) = $6\frac{1}{4}$ days