The 475 litres is filled in 19 hours by the two pipes when they are opened simultaneously.
Therefore, rate of water flow by A and B,
A + B = = 25 litres/hour………….(1)
Let pipe B was opened for t hours
Then, pipe A was opened for t + 5 hours
A x (t+5) + B x t = 475
(A + B) x t + 5A = 475
25t + 5A = 475
=) 5t + A = 95
Now, using option analysis
a) t = 10 =) 5 x 10 + A = 95 =) A = 45 (not possible from (1))
b) t = 14 =) 5 x 14 + A = 95 =) A = 25 (not possible from (1) as B can’t be zero)
c) t = 16 =) 5 x 16 + A = 95 =) A = 15
Correct Answer: 16 h