Good job, heureka.....here is how I solved it.....
Each hour, the candle that burns down more quickly loses 7/28 of its height.....and the other candle loses 4/28 of its height.....so......we want to know when.....
2 ( h - [7/28]h* t ) = (h - [4/28]h* t) where h is the original height and t is the time we are looking for, in hours........
2h - [14/28]h*t = h - [4/28]h*t
1h = [10/28]h * t divide through by h
1 = [10/28] * t
28/10 = t = 2.8 hours
