Buildings and bamboo poles are in proportion to their shadows at the same time, just like two right-angled triangles with parallel bases. The height-to-width ratio is 2/0.8, the shadow of the building is 7.2, and the height of the building is 2/0.8 and then multiplied by 7.2= 18, or an equation is made to set the height of the building to x meters, and the corresponding ratio is 2/0.8 = x/7.2.