(a) Torque is defined as rate of change of angular momentum
$\therefore$ $\tau = \frac{dJ}{dt} = \frac{d(I\omega)}{dt}$
Given, $\tau = 40\ Nm,\ I = 5\ kgm^{2},\ \omega = 24\ rads^{- 1}$
$dt = \frac{d(I\omega)}{\tau} = \frac{5 \times 24}{40} = 3\ $s