Speed upstream $u = \frac{50}{5} = 10\text{ km/h}$.
Speed downstream $v = \frac{49}{7} = 7\text{ km/h}$.
Speed of boat in still/standing water:

Total distance to be covered = Sum of lengths of the two trains:

Relative speed when moving towards each other:

In 3 hours, portion filled by (A + B + C) $= \frac{3}{9} = \frac{1}{3}$.
Remaining part $= 1 - \frac{1}{3} = \frac{2}{3}$.
(A + B) fill $\frac{2}{3}$ tank in 12 hours $\implies$ (A + B) take $12 \times \frac{3}{2} = 18\text{ hours}$ to fill the full tank.
Work rate of C alone:

Step-by-step solution for net rate of filling:

• Filling rate of inlet pipe $= +\frac{1}{30}\text{ tank/min}$.
• Emptying rate of leak $= -\frac{1}{60}\text{ tank/min}$.
• Net filling rate per minute:
  $\text{Net Rate} = \frac{1}{30} - \frac{1}{60} = \frac{2 - 1}{60} = \frac{1}{60}\text{ tank/min}$.
• Time required to fill the tank completely:
  $\text{Time} = \frac{1}{1/60} = 60\text{ minutes}$.