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
 

From fig, candle floating in liquid
wt.of candle = uptrust on candle due to liquid
$\begin{array}{l}{\mathrm{V}}_{\mathrm{b}}{\mathrm{d}}_{\mathrm{b}}\mathrm{g}={\mathrm{V}}_{\text{in }}{\mathrm{d}}_{\mathrm{L}}\mathrm{g}=\left(\frac{{\mathrm{V}}_{\mathrm{b}}}{2}\right){\mathrm{d}}_{\mathrm{L}}\mathrm{g}\\ {\mathrm{d}}_{\mathrm{b}}=\frac{{\mathrm{d}}_{\mathrm{L}}}{2}\Rightarrow {\mathrm{d}}_{\mathrm{b}}=2\left({\mathrm{d}}_{\mathrm{L}}\right)\end{array}$
For every 1 hr , decrease in length is '2cm'
i.e length of candle inside & outside is same
Top of candle fall at rate of 1cm/hr