Correct Answer - Option 2 : 2
Calculation:
Let t be the time to fill the tank and k is the part of the tank filled by a single pipe in one hour (efficiency).
Total tank volume = 10 × t × k = 10kt ---(1)
p pipes were opened
time taken to fill the tank = (125/100) × t = 1.25 t
Total tank volume = p × t new × k = 1.25pkt ---(2)
(1) = (2) ⇒ 10kt = 1.25pkt
⇒ p = 8
Number of pipes didn't open was 2.