$$H = {{{V^2}} \over R}\,.\,\Delta t$$

$$ \Rightarrow H = {{{V^2}} \over {{R_1}}}\,.\,20 = {{{V^2}} \over {{R_2}}}\,.\,60$$ ..... (i)

और, $$H = {{{V^2}} \over {\left[ {{{{R_1}{R_2}} \over {{R_1} + {R_2}}}} \right]}}\,.\,\Delta t$$

$$ = {4 \over 3}\,.\,{{{V^2}} \over {{R_1}}}\,.\,\Delta t$$ [$$\because$$ $${R_2} = 3{R_1}$$]

$$ \Rightarrow \Delta t = 15$$