Question 140
Filling a pool - An empty pool was filled with water by two taps $A$ and $B$, both with constant flow. For 4 hours, the two faucets were left open and filled $50\%$ from the pool. Then the tap $B$ was closed and for 2 hours the tap $A$ filled the pool volume $15\%$. After this period the tap $A$ was closed and the faucet $B$ opened. How long did this faucet have to stay open for it to finish filling the pool by itself?