#### Answer

(a) $4\pi$ radians.
(b) $\frac{2\pi}{3}$ radians

#### Work Step by Step

(a) The hour hand on a clock makes two complete rotations in $24$ hours as one complete rotation represents $12$ hours. One complete rotation is represented by $360^{\circ}$ or $2\pi$ radians. Therefore, two complete rotations represent $2(2\pi)=4\pi$ radians.
(b) Since one complete rotation of $12$ hours represents $2\pi$ radians, $4$ hours represent one-third of this time period. Therefore, $4$ hours represent $\frac{1}{3}\times2\pi=\frac{2\pi}{3}$ radians.