Pipe A alone fills a tank in 30 minutes and pipe B alone in 45 minutes. A is opened first. After how many minutes should B be opened so that the tank is full exactly 21 minutes after A was opened?

Question

Accepted Answer

7.5 min. A in 21 min: 21/30=0.7. B must fill 0.3, i.e. run 0.3×45=13.5 min, so it starts at 21-13.5=7.5 min. (13.5 min is how long B runs.)

Pipe A alone fills a tank in 30 minutes and pipe B alone in 45 minutes. A is opened first. After how many minutes should B be opened so that the tank is full exactly 21 minutes after A was opened?

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