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In such questions, it always helps to treat it like a distance-time-speed problem. In other words, the rate, amount of work and time play the respective roles of speed, distance and time in questions of motion.

Assign signs to the rate based on whether the tank is being filled or emptied. For instance, in this question:

Time taken by pipe for filling tank = Amount of work/Rate = Capacity of tank/Rate = 200/5 = 40 hrs
Let the rate at which the tank leaks = x litres/hr

When both are at work (pipe and leak), the effective rate of filling the tank = Rate of pipe (filling) - Rate of leak (emptying) = 5 - x litres/hr
When both are at work, time taken to fill the tank = 40 + 80 = 120 hrs

We know that, Time = Work /Rate

Plugging in the respective values, we have:

120 = 200/(5 - x)
==> 120(5 - x) = 200
==> 600 - 120x = 200
==> 600 - 200 = 120x
==> x = 3.33 litres/hr

Answer = (C)

Hope this helps.
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