Initial kinetic energy \(\left( k _{ i }\right)=\frac{1}{2} mu ^{2}\)

\(=\frac{1}{2} \times 0.5 \times 20 \times 20=100 J\)

After deflection it moves with \(5 \%\) of \(k _{ i }\)

\(\therefore k _{ f }=\frac{5}{100} \times k _{ i } \Rightarrow \frac{5}{100} \times 100\)

\(\Rightarrow k_{ f }=5 J\)

Now, let the final speed be 'v' \(m / s\), then :

\(k_{ f }=5=\frac{1}{2} mv ^{2}\)

\(\Rightarrow v ^{2}=20\)

\(\Rightarrow v =\sqrt{20}=4.47 m / s\)