What's the area of a rectangle whose perimeter is 30 cm and the longest side is 4 times longer than the shortest?

Let the shortest side = X and the longer = 4X

perimeter = (length + width) * 2

30 = (4X + X) * 2

30 = 8X + 2X

30 = 10X divide 10 to both sides

3 = X

the longer side = 4X = 4 * 3 = 12 cm

the shortest side = X = 3 cm

area = length * width

area = 3*12 = 36 cm^2

do you understand?