the height of the smaller rectangle, let us say is 5
5 x 4 = 20
(multiply height of rectangle to scale factor)
the length is 4
4 x 4 = 16
(multiply length of rectangle to scale factor)
to find larger factor, divide smaller with larger
16/20 = 1/4
scale factor should be 1/4
multiply 20 with 4
perimeter of larger should be 80
hope this helps