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

15 days

A = 30 days,

Given, 

 B is 25% more efficient than A and C is 20% more efficient than B.

Therefore,    A : B : C 

Efficiency     4 : 5 : 6

⇒ Total work = 4 x 30 = 120 units  ..(Efficiency × Days = Total work)

⇒ A + B + C worked for 3 days = (4 + 5 + 6) x 3 = (15) x 3 = 45 units.

⇒ Remaining work = 120 - 45 = 75 units.

⇒ Time required by B to complete 75 units = \(\frac{75}{5}\) = 15 days.  ..(\(\frac{Work}{Efficiency}\) = Time)