A can complete a certain work in 30 days. B is 25% more efficient than A and C is 20% moreefficient than B. They all worked together for 3 days. B alone will complete the remaining work in:

Let the efficiency of A be x.

Efficiency of B = 25% more efficient than A = 1.25x

Efficiency of C = 20% more efficient than B = $$1.25x \times \frac{120}{100}$$ = 1.5x

Efficiency ratio of A, B and C = x: 1.25x : 1.5x = 4 : 5 : 6

Total work = efficiency $$\times$$ time = 4 $$\times$$ 30 = 120 units

3 days work = (4 + 5 + 6) $$\times 3 = 15\times$$ 3 = 45 units

Remaining work = 120 - 45 = 75 units

Time taken to complete work by B = 75/5 = 15 days