A proton $($mass $1$ u $)$ is shot toward an unknown target nucleus at a speed of $2\mathrm{.}25{10}^6\frac{m}{s}.$ The proton rebounds with its speed reduced by $25\%$ while the target nucleus acquires a speed of $2\mathrm{.}46{10}^5\frac{m}{s}.$

Part A

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

Express your answer in atomic mass units.

$m=,u$

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