A pool has a volume of $4000 \mathrm{~L}$. Sheila starts filling the empty pool with water at a rate of $20 \mathrm{~L} / \mathrm{min}$. The pool springs a leak after 20 minutes and water leaks out at $2 \mathrm{~L} / \mathrm{min}$. Beginning from the time when Sheila starts filling the empty pool, how long does it take until the pool is completely full?
(A) 3 hours
(D) 4 hours 20 minutes
(B) 3 hours 40 minutes
(E) 3 hours 20 minutes
(C) 4 hours
In the first 20 minutes, Sheila fills the pool at a rate of $20 \mathrm{~L} / \mathrm{min}$ and thus adds $20 \times 20=400 \mathrm{~L}$ of water to the pool.

At this time, the pool needs $4000-400=3600 \mathrm{~L}$ of water to be full.

After filling for 20 minutes, water begins to leak out of the pool at a rate of $2 \mathrm{~L} / \mathrm{min}$.

Since water is still entering the pool at a rate of $20 \mathrm{~L} / \mathrm{min}$, then the net result is that the pool is filling at a rate of $20-2=18 \mathrm{~L} / \mathrm{min}$.

Since the pool needs $3600 \mathrm{~L}$ of water to be full and is filling at a rate of $18 \mathrm{~L} / \mathrm{min}$, then it will take an additional $3600 \div 18=200$ minutes before the pool is full of water.

Thus, the total time needed to fill the pool is $20+200=220$ minutes or 3 hours and 40 minutes.

ANSWER: (B)