A, B and C can complete a piece of work separately in 10, 20 and 40 days, respectively. In how many days will the work be completed if A is assisted by both B and C every third day?

A's three days work = 4 × 3 = 12
B and C one day work = 3 × 1 = 3
A, B, C three days work = 15 unit
A, B, C six days work = 30 unit
7^th day work = 4 unit
8^th day work = 4 unit
Total = 38 unit
Work left 2 unit
7 units in 1 days
2 units in $$\frac{2}{7}$$ days
Total time = $$8\frac{2}{7}$$ days