Question 60414
Lets work this problem two ways:
First, let x=the number of hours that have elapsed at the time when the candles are the same height.
At this time, the 12cm candle is 12-3x cm tall and
the 10cm candle is 10-2x cm tall.  If they are the same height, then these two quantities must be equal and our equation is:
12-3x=10-2x; and solving, we have x=2 hours

This problem can also be worked graphically: Draw a 12cm rectangle and a 10cm rectangle beside each other.  Put lines across the 12cm rectangle at 3cm intervals and put lines across the 10cm rectangle at 2cm intervals and note where the lines coincide.

Hope this helps -----ptaylor