Ratio of replica to real

What is the ratio of my replica to real size?  My drawing of a billboard is 5 inches by 10 inches, and it is a replica of a billboard that is 10 feet by 20 feet... what is the ratio between these two sizes? How do I calculate that?

Change the feet to inches

Drawing = 5 in. x 10 in.
Actual = 10 ft x 20 ft -----> 120 in. x 240 in  {changed feet to inches}

5/120 x 10/240   {width and length of drawing over width and length of actual}
= 1/24 x 1/24   {ratio of widths and lengths, in inches, reduced}

The ratio is 1:24

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