If it takes A four days to dig a certain ditch, whereas B can dig it in 8 days and A, B, C together can dig it in 2 2\7 days, how long C alone would take to dig it?

Explanation:

According to the question,

A’s one day work = 1\4

B’s one day work = 1\8

(A + B)’s one day work

= 1\4+1\8 = 3\8

Similarly,

(A + B + C)’s one day work = 7\16

∴ C’s one day work

= 7\16−3\8 = 1\16

Therefore, C can complete the whole work in 16 days.