Think of it like a triangle.
The flagpole is vertical and is at 90° to the ground. This makes a right-angle to the ground, which is flat.
Since we have two sides of the triangle already, and they are in a ratio of ?? : 16 : 20 = ?? : 4 : 5, that means that because the triangle is a right-angled triangle, the third and last side must be 3 to complete the ratio! (This is called 'Pythagoras')
Since the other sides were divided by 4 to get the simple ratio, we must multiply 3 to get the actual ratio! 3 × 4 = 12 ft tall flagpole.