Question

If my scale drawing is 1/4in:12ft what's the actual length for 8ft, 30ft, 18ft, and 20ft?

Answer 1

if 1/4 in:12ft, that is the same as saying 1in:48 ft

so take the 'actual length' divide it by 48, and you will get how many inches for the drawing:

like this:
8ft / 48ft = 0.167 inches

Answer 2

12 ft = 12 ft * 12 in./ft = 144 in.
1/4 in. : 12 ft is the same as 1/4 in. : 144 in.
1/4 in. : 144 in. is the same as 1 in. : 576 in.

That means, the scale drawing is 1/576 of the real size.
Take each real measure and multiply by 12 to get inches, then divide by 576 to get the scale measure in inches.