$5$C $(10$ Points$)$: If the initial kinetic energy of the fission fragment $A{g}^{121}(1)$ is $71\mathrm{.}8$ MeV $,$ what is the initial kinetic energy of the other fission fragment $(2)?$ Assume that the momentum carried by the prompt neutrons is negligible.

Hint: Conservation of momentum indicates that the momentum of fission fragment $1$ is equal $($and opposite$)$ to the momentum of fission fragment $2.$ Thus $m_1{v}_1=m_2{v}_2,$ where $v$ is the fission fragment velocity $($magnitude$).$ In collision problems such as this, both energy and momentum have to conserved independently $($non$-$relativistic$).$