Given, 295 m was mined out of which first 5 days – planned.
a) 30 * 5 + (30 + 5) * (300/30 – 5 – 4) = 150 + 35 * 1 = 185 ≠ 295
b) 12.5 * 5 + 17.5 * (300/12.5– 5 – 4) = 62.5 + 17.5 * 15 = 325 ≠ 295
c) 15 * 5 + 20 * (300/15– 5 – 4)1 = 75 + 20 * 11 = 295

A is kept open for all t minutes and fills one-third the tank. Or, A should be able to fill the entire tank in '3t' minutes.
A and B together can fill the tank completely in t minutes. A alone can fill it in 3t minutes.
A and B together can fill 1/t of the tank in a minute. A alone can fill 1/3t of the tank in a minute. So, in a minute, B can fill 1/t – 1/3t = 2/3t  

 Or, B takes 3t/2 minutes to fill an entire tank.
To fill one-third the tank, B will take t/2 minutes. B is kept open for t - 10 minutes.
t/2 = t - 10, t = 20 minutes.
A takes 60 minutes to fill the entire tank, B takes 30 minutes to fill the entire tank. A is kept open for all 20 minutes. B is kept open for 10 minutes.
C, which is kept open for 8 minutes also fills one-third the tank. Or, C alone can fill the tank in 24 minutes.

If the digits of N are distinct, M and N are not palindromes So only the latter 8 combinations are possible.