A proton $($mass $1$ uu$)$ is shot toward an unknown target nucleus at a speed of $2\mathrm{.}35106$ m$/$sm$/$s$.$ The proton rebounds with its speed reduced by $25\%\%$ while the target nucleus acquires a speed of $2\mathrm{.}57105$ m$/$sm$/$s$.$

Part A

What is the mass, in atomic mass units, of the target nucleus?

Express your answer in atomic mass units.

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