A pool has a capacity of 4,000 litres. Mr Osborne starts filling the empty pool with water at a rate of 20L/min.
The pool springs a leak after 20 minutes and water leaks out at 2L/min. Beginning from the time when Mr. Osborne
starts filling the empty pool, how long does it take until the pool is completely full?

A. 4 hours
B. 3 hours and 20 minutes
C. 3 hours and 40 minutes
D. 4 hours and 20 minutes
E. 3 hours

After 20 minutes, it fills 20L but leaks 2L -> the real filling rate at that time is 18L/min

The first 20 minutes, it fills : 20 x 20 = 400 litres -> it's left with 3600 litres

After that, the filling rate is 18L/min -> it will take : 3600 : 18 = 200 (minutes) to fill 3600 litres

So in total, it will take 200 + 20 = 220 minutes = 3 hours 40 minutes.

Option C is the correct one.

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