Correct Answer - `10 minutes `

`H = (V^(2)t_(1))/(R_(1)) = (V^(2)t_(2))/(R_(2)) or (R_(1))/(R_(2)) = (l_(1))/(l_(2))` ….(i)

But ` R prop 1, so (R_(1))/(R_(2)) = (l_(1))/("l_(2))`….(ii)

Now `l_(2) = (2//3) l_(1)`

From (i) and (ii)

`(l_(1))/(l_(2)) = (r_(1))/(r_(2)) or (3)/(2) = (15)/(t_(2)) or l_(2) = 10 minutes `