GMAT
题目
Pipes P and Q can fill a tank in 12 minutes and 16 minutes respectively. Both are kept open for X minute(s) and then Q is closed and P fills the rest of the tank in 5 minutes. The time X after which Q was closed is –
解析
设水箱的容量为48单位。
因此，P管的效率为4单位/分钟，Q管的效率为3单位/分钟。
P、Q两管的合效率为7单位/分钟。
两管同时打开\( x \)分钟，这段时间内注入的水量为\( 7x \)单位。
之后关闭Q管，P管单独注满剩余水箱用了5分钟。
据此列方程：
\[ 4×5 + 7x = 48 \]
即 \( 7x = 28 \)
解得 \( x = 4 \)，答案是（C）
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