The acceleration from zero to $5 \,s$ is $a=\frac{0-20}{5-0}=\frac{-20}{5}=-4 \,ms ^{-2}$
From $5 \,s$ to $10 \,s$
$a=\frac{20-0}{10-5}=4 \,ms ^{-2}$
$a=\frac{\text { Total change in velocity }}{\text { Time }}$
$=\frac{20-20}{10-0}=0 \,ms ^{-2}$
$\left[ g =10 \,ms ^{-2}\right.$ લો.$]$