Answer
$0.87c$
Work Step by Step
We know that
$v=c\sqrt{1-(\frac{\Delta t_{\circ}}{\Delta t})^2}$
We plug in the known values to obtain:
$v=c\sqrt{1-(\frac{7.5s}{15s})^2}$
$v=c\sqrt{0.75}$
$v=0.87c$
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