a
According to \(Newton's\) law of coolling,

\(\left( {\frac{{{\theta _1} - {\theta _2}}}{t}} \right) = K\left( {\frac{{{\theta _1} + {\theta _2}}}{2} - {\theta _0}} \right)\)

\(\left( {\frac{{60 - 50}}{{10}}} \right) = K\left( {\frac{{60 + 50}}{2} - 25} \right)\,\,\,\,\,\,\,\,\,...\left( i \right)\)

\(and,\left( {\frac{{50 - \theta }}{{10}}} \right) = K\left( {\frac{{50 + \theta }}{2} - 25} \right)\,\,\,\,\,\,\,\,\,\,\,...\left( {ii} \right)\)

Dividing eq. \((i)\) by \((ii)\),

\(\frac{{10}}{{\left( {50 - \theta } \right)}} = \frac{{60}}{\theta } \Rightarrow \theta  = {42.85^ \circ }C \cong {43^ \circ }C\)