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KE is increased by 400 Joules.

KE=\frac{P^2}{2m},\ \ \ P=\sqrt{2m*KE}\\\\P_2-P1=\sqrt{2m}(\sqrt{KE_2}-\sqrt{KE_1})\\\\ \frac{P_2-P1}{P1}=\frac{(\sqrt{KE_2}-\sqrt{KE_1})}{\sqrt{KE_1}}=\sqrt{\frac{KE_2}{KE_1}}-1\\\\ \%increase\ in\ momentum=100*\ [ \sqrt{\frac{KE_1+400}{KE_1}} - 1]\\\\=100*\ [ \sqrt{1+\frac{400}{KE_1}} - 1]

If you know the momentum, or velocity or mass then you can numerically calculate it.