Two candles with different thicknesses and lengths, the long one can burn for 7 hours, and the short one can burn for 10 hours, then after 4 hours of lighting, the lengths of the two candles are the same, if the length of the original long candle is a, the original short candle How long is it.
两支粗细、长短都不同的蜡烛，长的能燃烧7小时，短的能燃烧10小时，则点燃4小时后，两只蜡烛的长度相同，若设原来长蜡烛的长为a，原来短蜡烛的长是多少．
<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<result>5/7 = around 0.714286</result>