The mirror is 10 times as far to the dinosaur as it is to Amir: 40 ft = 10×4 ft.
The triangles shown in the diagram are similar, so the height of the dinosaur will be 10 times Amir's eye height. In feet, Amir's eye height is 5 6/12 feet, or 5.5 feet. Ten times that is 55 feet.
