Varies directly as the square

Question

The area of a projected picture on a movie screen varies directly as the square of the distance from the projector to the screen.  If the distance of 20 ft produces a picture with an area of 6 [c] square feet, what distance produces an area of 1[c]0 sq ft?

Answer 1

a = d²k

6 = 20²k 

3/200 = k

 

10 = 3d²/200

2000 = 3d²

666.67 ≈ d²

25.8 ≈ d