The was a 30% decrease in area.
You started with 40 sq ft and ended up with 28 sq ft. So you lost 40 - 28 = 12 sq ft.
Now you need to figure out what percentage of 40 is 12. For that you just simply divide
12 / 40 = 0.30
And to convert 0.30 from a decimal to a percentage, just multiply by 100.
0.30 * 100 = 30%
