Answer
$x=24$
Work Step by Step
To solve the proportion, we put the product of the means equal to the product of the extremes as follows:
$$\frac{3}{x}=\frac{8}{64}\Longrightarrow x\times 8= 3\times 64\Longrightarrow x=\frac{3\times 64}{8}=24 .$$
