Explanation

(a) If a motor of $12\ HP$ works for 10 days at the rate of 8 h/day then
Energy consumption = $Power\ \times time$
$= 12 \times 746\frac{J}{s} \times (80 \times 60 \times 60)s$
$= 12 \times 746 \times 80 \times 60 \times 60J = 2.5 \times 10^{9}J$
Rate of energy$\ = 50\frac{paise}{kWh}$
$\therefore 3.6 \times 10^{6}J\ $energy cost $= Rs\ 0.5$
$\therefore 2.5 \times 10^{9}J\ $energy cost $= \frac{2.5 \times 10^{9}}{2 \times 3.6 \times 10^{6}} = Rs\ 347$