A and B can do a job in 10 days and 5 days respectively. They worked together for two days, after which B was replaced by C and the work was finished in the next three days. How long will C alone take to finish 40% of the job?

Two days work of A and B = 3 × 2 = 6
Remaining work = 10 - 6 = 4
Now as per question
$$\eqalign{ & \frac{4}{{1 + {\text{C}}}} = 3 \cr & \frac{4}{3} = 1 + {\text{C}} \cr & {\text{C}} = \frac{1}{3} \cr} $$
So that 40% of total work done by $${\text{C}} = \frac{{10 \times 40\% }}{{\frac{1}{3}}} = 12$$