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 ?