After collision,

Applying momentum conservation,
mu=mv1+5mv2 ⇒u=v1+5v2...(1)
Coefficient of restitution, e=1=uv2−v1
∴u=v2−v1...(2)
Adding 2u=6v2 ⇒v2=3u and u=3u−v1
∴v1=3u−u=−32u
Percentage change in kinetic energy =21mu221mv12−21mu2×100
=u2v12−u2×100=u294u2−u2×100 =9−5×100 =55.6