Question 24
Robert bought two different candles. The ratio of the length of the shorter candle to the longer candle is $5:7$. It is known that the longer candle, when lighted, will burn out completely in $3.5$ hours while the shorter candle, when lighted, will burn out completely in $5$ hours. If the two candles are lighted at the same time, after how many hours will the length of the two candles be exactly equal?