A candle of diameter d is floating on a liquid in a cylindrical container of diameter D (D>>d) as shown in figure. If it is burning at the rate of 2cm/hour then the top of the candle will 

Density of candle is half of the density of water. So, it always remains half inside water and half outside. Thus after an hour, the length of candle would decrease by 2cm and half of it would remain in water and half outside. Thus the fall in the top of the candle would be 1cm.
Thus rate of fall of top of the candle is 1cm/h