Answer
a). $(3)75$% is the correct answer.
b). $7.5J$
Work Step by Step
a). $KE_{i}=\frac{1}{2}mv^{2}$
$KE_{f}=\frac{1}{2}m(\frac{v}{2})^{2}=\frac{1}{4}KE_{i}$
So, final KE is 25% of initial KE.
This means that 75% of initial KE was lost.
Hence (3) is the correct answer.
b). Loss in KE = $0.75\times\frac{1}{2}mv^{2}=0.75\times0.5\times0.2\times10^{2}=7.5J$
