C can complete a work in 20 days. If C works for 5 days and then D finishes the remaining work in 10 days, how long would D take to complete the entire work alone?
Answer: B. 30 days
C completes 5/20 = 1/4 of the work in 5 days, leaving 3/4 of the work. If D finishes this in 10 days, D's work rate is 3/4 in 10 days, so D would take 40 days to complete the entire work alone.