Explanation

(c) Given, $R_{300} = 0.3\Omega,\ R_{t} = 0.6\Omega$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ T = 300K = 27{^\circ}C$
Temperature coefficient of resistance,
$\ \ \ \ \ \ \ \ \ \alpha = 1.5 \times 10^{- 3}K^{- 1}\ $
$\therefore R_{300} = R_{0}(1 + \alpha \times 27)$
$\ 0.3 = R_{0}\left( 1 + 1.5 \times 10^{- 3} \times 27 \right)\ \ \ldots(i)$
Again, $R_{t} = R_{0}(1 + \alpha t)$
$0.6 = R_{0}\left( 1 + \times 10^{- 3} \times t \right)\ \ \ldots(ii)$
Dividing Eq. (ii) by Eq. (i), we get
$\frac{0.6}{0.3} = \frac{1 + 1.5 \times 10^{- 3}t}{1 + 1.5 \times 10^{- 3} \times 27\ }$
$\Longrightarrow 2\left( 1 + 1.5 \times 10^{- 3} \times 27 \right) = 1 + 1.5 \times 10^{- 3}t$
$\Longrightarrow 2 + 81 \times 10^{- 3} = 1 + 1.5 \times 10^{- 3}t$
$\Longrightarrow 2 + 0.081 = 1 + 1.5 \times 10^{- 3}t$
$\Longrightarrow t = \frac{1.081}{1.5 \times 10^{- 3}} = 720{^\circ}C = 993K$