If 1 inch = 1 foot then 1 foot = 1 inch.
With this, you can make a ratio of 1 foot:1 inch, or 1:1.
Starting with the first dimension, 27 feet, just change the 1 to 27 in the ratio.
27:?
To find how many inches this is in the scale drawing, find how much 1 had to be multiplied by to get to 27. This is 27, since anything times 1 is itself.
Just multiply the other side by 27 as well to get the answer for the first dimension.
1 • 27 = 27
So 27 feet = 27 inches in the scale drawing.
Now do the same for the second dimension.
1:1
20:?
1 • 20 = 20
20:20
The answer is that the scale drawing has dimensions of 27 inches by 20 inches if 1 inch = 1 foot is the scale
