A can do a piece of work in 10 days, and B can do the same work in 20 days. With the help of C, they finished the work in 4 days. C can do the work in how many days, working alone?
Answer: B
Their combined 4 day work = 4(1/10 + 1/15) = 12/20 = 3/5.
Remaining work = 1 – 3/5 = ⅖.
This means C did 2/5 work in 4 days, hence he can finish the complete work in 5/2 × 4 = 10 days.