Let the total distance be $D$ then
Avg speed $=\dfrac{D}{\text{total time taken}}$
Total time taken $=\left(\dfrac{D}{2} \times 60\right)+ \left(\dfrac{D}{4}\times 30\right)+ \left(\dfrac{D}{4}\times 10\right)$
$\text{Avg speed}- \dfrac{120}{5}=24.$
Correct Answer: $C$