The velocity given is minimum, just enough to complete verticle circle. At the top most point, tension of the string will be zero and gravitational force will provide the required centripetal force.
Therefore,
mg=Lm(VH)2⇒21m(VH)2=21gL
Apply energy conservation between point A and B, we get
21mVL2=21mVH2+mg(2L)
⇒VL=5gL
Also, VH=gL
Required ratio, (K.E)B(K.E)A=21m(gL)221m(5gL)2=15
