Let's suppose we need to find the height of a pole, but it is rather difficult on us to measure vertically let's consider the following alternative. 

Problem: You are standing close to a pole. At the same time of the day you cast a shadow of x meter and the pole of y meters. We know your height to be h meters, find the height T of the pole.

Solution: Using proportions to solve the problem,
              h/x=T/y=>T=(h*y)/x