Correct Answer - Option 3 : 150 days

**Given: **

A is twice efficient as of B and B is thrice efficient as of C.

Time taken to complete the whole work = 15 days

**Formula used:**

Efficiency = Total work/TIme

**Calculation:**

Efficiency of A and B = 2 : 1 ----(1)

Efficiency of B and C = 3 : 1 ----(2)

Eq. (1) × 3 and Eq. (2) × 1; then solve:

⇒ Efficiency of A : B : C = 6 : 3 : 1

Total efficiency = 6 + 3 + 1 = 10 units/day

⇒ Total work = 15 days × 10 units/day

⇒ Total work = 150 units

Time taken by C to complete the whole work = 150/1 = 150 days

**∴**** C complete the whole work in 150 days.**