\(\mathrm{~K}_{\mathrm{f}} =\frac{1}{2} \mathrm{~m}(40)^2\)

\(\% \text { loss }=\frac{\left|\mathrm{K}_{\mathrm{f}}-\mathrm{K}_{\mathrm{i}}\right|}{\mathrm{K}_{\mathrm{i}}} \times 100\)

\(=\frac{\left|\frac{1}{2} \mathrm{~m}(40)^2-\frac{1}{2} \mathrm{~m}(100)^2\right|}{\frac{1}{2} \mathrm{~m}(100)^2} \times 100\)

\(=\frac{|1600-100 \times 100|}{100}=84 \%\)