Question 514072
Thin candle burning rate is 30/3 = 10 cm/h

Thick candle burning rate = 30/5 = 6 cm/h

Thin candle burns 10-6 = 4 cm/h faster than thick candle

Thicker candle is lit at 7
thinner candle is lit at 7.30
so difference = 1/2 hour.

in 1/2 hour the thin candle would have burnt 10*1/2 = 5 cm

time both will have same height = difference in length when second candle is lighted / difference between burning rates
5/4 = 1.25 hours 

m.ananth@hotmail.ca