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 \(40 \,sec\)
= \(\frac{1}{2} \times 20 \times 3 + 20 \times 1\) \(= 30 + 20 = 50 \,m.\)