Part of the tank filled when all the three pipes are opened in 1 minute = \(\frac{1}{15}\)
Part of the tank filled when (A + B) are opened in 1 minute = \(\frac{1}{20}\)+\(\frac{1}{24}\)= \(\frac{6+5}{120}\) = \(\frac{11}{120}\)
\(\therefore\) Part of the tank emptied by C in 1 min. = \(\frac{11}{120}\) - \(\frac{1}{15}\) = \(\frac{1}{40}\)
\(\therefore\) Capacity of \(\frac{1}{40}\) part of the tank = 3 gallons
\(\therefore\) Capacity of the tank = (3 × 40) gallons = 120 gallons.