Let the mass, energy and the speed of the man be m,k2,Vman
and the mass, energy and speed of the boy be m2,k,Vboy
k2=12m(Vman)2 and k=12×m2×(Vboy)
⇒Vman.Vboy=1:2
Now k=12m(Vman+1)2
⇒14mV2boy=12m(Vman+1)2
mV2man=12m(Vman+1)2
2V2man=(Vman+1)2
√2Vman=√Vman+1
⇒Vman=√2+1