Energy radiated from a black body is given by

$U=\sigma \cdot A \cdot T^4 \cdot t$

where, $\sigma=$ Stefan's constant, $A=$ area, $T=$ absolute temperature and $t=$ time.

Now, ratio of energy collected in two given cases is

$\frac{U_2}{U_1} =\frac {(A / 2)(2 T)^4} {A T^4}=8$

Hence, ratio of temperature rise of water is

$\frac{m s \Delta T_2}{m s \Delta T_1}=\frac{U_2}{U_1}=8 .$

$\Rightarrow \quad \Delta T_2=8 \Delta T_1$

$\text { As, } \Delta T_1=1^{\circ} C \text { and } \Delta T_2=8^{\circ} C .$

So, temperature of water increases from $10^{\circ} C$ to $18^{\circ} C .$