Johnny has bought two candles of equal length but different diameters. One of the candles will burn up completely in 10 hours, while the other candle requires 40% more time to burn up completely. If the candles are lit at the same time, approximately how long will they burn before one of the candles is twice the length of the other?
1)7.8 hours
2)6.4 hours
3)5.6 hours
4)6 hours