Explanation

(a) Motor makes 600 revolution per minute
$\therefore n = 600\frac{revolution}{minute} = 10\frac{rev}{\sec}$
$\therefore$ Time required for one revolution $= \frac{1}{10}\ sec$
Energy required for one revolution = power$\ \times$ time
$= \frac{1}{4}\ \times 746 \times \frac{1}{10} = \frac{746}{40}\ J$
But work done $= 40\%$ of input
$= 40\% \times \frac{746}{40} = \frac{40}{100} \times \frac{746}{40} = 7.46\ J$