Answer
Please see the work below.
Work Step by Step
We know that
$60.0mL=0.0600L$
$mol=M\times L$
$mol\space MgCl_2=\frac{0.100mol\space MgCl_2}{1 L\space soln}\times 0.0600\space L\space soln=6.00\times 10^{-3}mol\space MgCl_2$
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