Correct Answer - Option 3 : 15 days
Given:
A alone can do the work in = 12 days
B's efficiency = 20% more than A's efficiency
A, B and C together finish the work in = 4 days
Concept used:
Total work = Efficiency × Number of days to finish the work
Calculations:
Let C's 1 day work be 1/C
A's 1 day work = 1/12
B's 1 day work = (120/100) × (1/12) = 1/10
(A + B + C)'s 1 day work = 1/4
⇒ (1/12) + (1/10) + (1/C) = 1/4
⇒ (11/60) + (1/C) = 15/60
⇒ 1/C = (15 – 11)/60
⇒ 1/C = 4/60
⇒ 1/C = 1/15
∴ C alone can finish the whole work in 15 days