By decreasing each dimension by 1 unit, the area of a rectangle decrease from 40 square feet (on the left) to 28 square feet (on the right). find the percent of decrease in area,

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%