The acceleration from zero to \(5 \,s\) is \(a=\frac{0-20}{5-0}=\frac{-20}{5}=-4 \,ms ^{-2}\)
From \(5 \,s\) to \(10 \,s\)
\(a=\frac{20-0}{10-5}=4 \,ms ^{-2}\)
\(a=\frac{\text { Total change in velocity }}{\text { Time }}\)
\(=\frac{20-20}{10-0}=0 \,ms ^{-2}\)