Point $B$ is the image of point $A$ under a dilation with center $O$. If $OB = 30$, and the scale factor of the dilation is $5$, then what is $OA$?

You can think of a dilation as multiplying lengths by the dilation factor.

The scale factor is 5, and the image (end result) of the dilation is 30.

To get the original length OA, simply ask: What multiplied by 5 is 30?

The answer is 6.