The ratio of their perimeters is still 7:20. The poster is a rectangle shape. If the length of each side of a poster is proportional to the other by a certain ratio, the sum of the lengths of all sides is also proportional by the same ratio. You can suppose the lengths of four sides of poster A are 7a, 7b, 7c, 7d, so poster B has 20a, 20b, 20c, 20d. (7a+7b+7c+7d)/(20a+20b+20c+20d)=7(a+b+c+d)/20(a+b+c+d)=7/20.
