Assuming S=1 you would take the log of 8.5 and the log of 5.3 and subtract then to get the difference in intensity. If you put that in a calculator you will get roughly 0.205143056
Both legs of an isosceles triangle are the same length, so knowing that when the shadow of the triangle was the same height as the triangle, then he knew that the shadow of an object would also be the same as it's height.
