A quick point here - from the way the question is worded, we don't even know "when" the first tap stops, as "when" begins a new sentence:
Two identical taps fill 2/5 of a tank in 20 minutes. When one tap stops, how long will it take the other to finish the tap?
Those two sentences are two completely separate thoughts - the first gives us an indication of the combined rate, and the second asks a question. But we don't know at which point the first tap stops - it could be after the tank is half full, or after 30 minutes, or at any other point. That next step is what's missing from this question.
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