Capacitor

Ans:a
According to my working, K=1/5. Please explain how to solve this sum & where did I go wrong? Please show working.

@anag c2 is c1/k as q is conserved and
u=(1/2 )*v^2/c =1/2 *q^2 *c

@kdsdhaniksamba How is it possible?
So will K=0.2=1/5 (according to my working)
or K=5(according to answer given)?

@anag
Forgot the upper one it was typing mistake