b
(b) Between time interval $20\, sec$ to $40\, sec$, there is non-zero acceleration and retardation. Hence distance travelled during this interval
= Area between time interval $20\, sec$ to $40 \,sec$
= $\frac{1}{2} \times 20 \times 3 + 20 \times 1$ $= 30 + 20 = 50 \,m.$