Question

given that the perimeter of a rectangle is 64cm and its length is 20cm find its area

Answer 1

240 cm. The total perimeter minus twice the length. That would be 24. Then divide 24 by 2 to get your width. Then multiply the length times the width or 20 times 12

Answer 2

Because this is a rectangle you know that it has two 20cm lengths. To find the area first find the two widths, to do that create an equation and solve for width: 20+20+2w=64, 40+2w=64, 2w=24, w=12; Now that you now the width, find the area: 20×12=240cm².
The area is 240cm²!