\(k =\frac{1}{2} \times 5 \times 400=5 \times 200=1000\,J\)
\(\frac{\Delta k }{2 k }=\frac{\Delta m }{ m }+\frac{2 \Delta v }{ v }=\frac{0.5}{5}+\frac{2 \times 0.4}{20}\)
\(\Delta k =1000\left(\frac{1}{10}+\frac{4}{100}\right)=1000\left(\frac{10+4}{100}\right)=140\,J\)