Proportional Candles

You have two candles of the same length, but the candles contain wax that melts at different rates. Assume the wax is uniform throughout the candle so that the length decreases at a constant rate. One of the candles melts in $n$ hours, while the other candle melts in $m=an$ hours, where $a > 1$. After how many hours will the second candle be $k$ times the length of the first candle, where $k \geq 1$? Report the answer when $n = 30, k = 2,$ and $a = 2$.