(a) Power is defined as the rate of doing work, it is the work done in unit time. The SI unit of power is Watt (W) which is joules per second $\left(\mathrm{J}\backslash /\mathrm{}\mathit{s}\right)$.
(b) Given : Potential difference $=\mathit{V}=5\mathrm{V}$

(i) Power $=\mathrm{V}\times \mathrm{I}=5\times 0.5=5\times \frac{5}{10}=2.5\mathrm{W}$
(ii) Resistance $=\mathrm{R}=$ ?
By Ohm's law: $\mathit{V}=\mathit{I}\mathit{R}$
$\mathit{R}=\frac{\mathit{V}}{\mathit{I}}$
$\mathit{R}=\frac{5}{0.5}$
$=\frac{50}{5}=10\mathrm{ohms}$
(iii) Energy consumed when it is lighted for $2\frac{1}{2}$ hours
Energy $=$ Power $\times$ Time
$\mathit{t}=2.5\times 60\times 60\mathrm{}\mathit{s}\mathit{e}\mathit{c}.$
So,

$=22500\mathrm{Joules}∕\mathrm{}\mathit{s}\mathit{e}\mathit{c}.$