b
$\frac{\Delta Q }{\Delta t }=- K \left( T - T _0\right)$
$\frac{\Delta Q }{\Delta t }=- K \left( T _{ avg }- T _0\right)$
$(i)$ $\frac{ ms \times 12}{2}=- K \left(\frac{98+86}{2}-22\right)$
$6=-\frac{ K }{ ms }\left[\frac{98+86}{2}-22\right]$
$6=-\frac{ K }{ ms }[70]..........(i)$
$(ii)$ $\frac{ ms \times 6}{\Delta t }=- K \left(\frac{75+69}{2}-22\right)$ $\frac{6}{\Delta t }=-\frac{ K }{ ms }(50) \ldots \ldots \ldots \ldots$ (ii)
$(ii)$ $\div$ $(i)$
$\frac{6}{\Delta t (6)}=\frac{50}{70}$
$\Delta t =\frac{7}{5}=1.4\,min$