Since the candle is melting, it is getting shorter over time. Think about this on a graph of height vs. time. As the time increases, the height would decrease. What correlation does this represent?
If the candle melts 1 inch every 30 minutes, and the candle was originally 12 inches, how many 30 minute periods would it take for the candle to melt completely?
Another way you could look at it is through an algebraic expression: 12 − 2x = 0
The candle is originally 12 inches, but loses 2 inches every hour. After how many hours does it melt completely and burn out?