1. Let original dimensions be W1 and L1
New dimensions designated as W2 and L2
Equation: W2/W1=L2/L1 ..... A equal proportion
Thus...
(1) W2/5=12/8. (2) W2/5=20/8
W2= 5(12/8) W2=5(20/8)
W2=5(1.5) W2=5(2.5)
W2=7.5" W2=12.5"
(3) W2/5=10/8
W2=5(10/8)
W2=5(1.25)
W2=6.25"