Three pipes A, B and C can fill a tank in 10 hours. After working at it together for 3 hours, C is closed and A and B can fill the remaining part in 14 hours. How much time taken by C to fill the tank alone?

Three pipes A, B, and C can fill a tank in 8 hours. A, B, and C’s 1 hour work=1/10
A, B and C's 3 hour work= 3/10 Remaining work= 1 – (3/10) = 7/10

The remaining part will be filled by A and B in 14 hours. Then,
⇒ (7/10) × (A + B) = 14
⇒ (A + B)'s whole work= 14 × (10/7)
= 20 hr (A + B)'s 1-hour work
= 1/20

A, B, and C's 1-hour work = 1/10
C's 1 hour work= (A + B + C) – (A + B)
⇒ (1/10) – (1/20)
⇒ 1/20
∴ C can fill the tank in 20 hours.