Answer
$x=6.5$
Work Step by Step
We can set the cross products of the given proportion equal to each other and then solve for the unknown variable.
$7.8\times x=3.9\times13$
$7.8x=50.7$
Divide both sides by 7.8.
$\frac{7.8x}{7.8}=\frac{50.7}{7.8}$
$x=6.5$