Four little black little imps drew a drawing in black ink for four hours. If the first little devil drew twice as fast, and the second half as slowly, it would take them the same amount of time; if, on the contrary, the first one had drawn twice as slow, and the second twice as fast, then they would have finished it in two hours and forty minutes. How long would it take the first three imps to draw the drawing without the help of the fourth?
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