find the ratio between the two known factors, in this case perimeter
64/48 = 1.33 so rect 2 is 1.33 larger than rect 1
now multiply both the sides by 1.333
15 x 1.333 = 20 , 9 x 1.333 = 12 so those are your sides
double check 20 + 20 + 12 + 12 = 64
so then area = 20 x 12 = 240
