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You have 2 candles of same height but of different widths. One takes 4 hours to burn all the way while the other takes 7 hours. Assuming they burn at steady rates, how long will it take before one candle is twice as tall as the other?

Answer: 2 hours 48 minutes.

Say the initial height of the candles is h, the burn rates are h/4 and h/7 respectively.

After time t, the heights of both candles are (h - h/4 * t) and (h - h/7 * t) respectively. We know that

2 * (h - h/4 * t) = (h - h/7 * t)

Therefore

t = 2.8 hours or 2 hours 48 minutes

This is too easy...

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