For one hour, the thick candle burns 1/5 and the thin candle burns 1/3.5. Within two hours, it burned 2/5 and 2/3.5 respectively, leaving 3/5 and 2.5/3.5 respectively. This time is equal.

Therefore, the ratio of the original length of thin candle to thick candle is 2.5/3.5: 3/5 = 25: 2 1.