Dear candidate, since it's elastic collision so by law of conservation of momentum -
mv0=mv2−mv1
12m(V1) ^2=0.36×12m(V0) ^2
v1=0.6v0
12m(V2)^ 2×0.64×12m(V0) ^2
V2=sqrt(mM) ×0.8V0
mV0=sqrt(mM)×0.8V0−m×0.6V0
⇒1.6m=0.8sqrt(mM)
4m^2=mM
Therefore the value of mass is 4
All the best
Hope this helps
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