Three pipes A, B and C can fill a cistern in 12, 18 and 24 minutes respectively. If all the pipes are opened for 4 minutes, then what fraction of the water left to be filled.

\(\frac{5}{18}\)

 Now total efficiency = 6 + 4 +3 = 13

In 4 minutes tank filled = 13 × 4 = 52 ltr

Remaining part of tank = 72 - 52 = 20

Fraction of part left to be filled = \(\frac{20}{72}\) = \(\frac{5}{18}\)