Explanation

(d) According to Newton’s law of cooling
$\frac{\theta_{2} - \theta_{1}}{t} = K\left\lbrack \frac{\theta_{1} + \theta_{2}}{2} - \theta_{s} \right\rbrack$
Where, $\theta_{s}$ is the temperature of the surroundings.
$\frac{60 - 50}{10} = K\left\lbrack \frac{60 + 50}{2} - \theta_{s} \right\rbrack$
$1 = K\lbrack 55 - \theta_{s}\rbrack$ …(i)
Similarly, $\frac{50 - 42}{10} = K(46 - \theta_{s})$
$\frac{8}{10} = K(46 - \theta_{s})$ …(ii)
Dividing Eq. (i) by Eq. (ii), we get
$\frac{10}{8} = \frac{K(55 - \theta_{s})}{K(46 - \theta_{s})}$
$\Rightarrow$ $\theta_{s} = 10℃$