Step-by-step explanation:
a proportional or similar shadow uses the same factor for each line/dimension.
so, since the height of the proportional rectangle is 4 times longer than the height of the original rectangle, then also the width must be 4 times longer.
heightP = 12 cm = f × height = f × 3 cm
f = 12/3 = 4
widthP = f × width = 4 × 5 = 20 cm
