Bottom of a lake is at $0^{\circ} C$ and atmospheric temperature is $-20^{\circ} C$. If $1 cm$ ice is formed on the surface in $24 \,h$, then time taken to form next $1 \,cm$ of ice is ......... $h$
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(b)
Time intervals to change thickness from 0 to $x$ from $x$ to $2 x$ are in ratio of $1: 3: 5: 7 \ldots \ldots$
$\therefore t_1:t_2=1: 3$
$=24: 24 \times 3$
$\Rightarrow t_2=72 \text { hours }$
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