A shadow 15 yards long is cast by a fir tree that is 8 yards tall. If a lookout tower close by casts a shadow 45 yards long, how tall is the lookout tower?

You can set up two ratios (a proportion) that relate the lookout to its shadow.

Height of tower / shadow length = height of tree / shadow lenght

Tree

height = 8 yards.

Shadow length = 15 yards.

Tower

Height of tower = ?

Shadow = 45 yards

8/15 = x / 45 Cross multiply

8 * 45 = 15 x multiply the left

360 = 15 x Divide by 15

360 / 15 = x

x = 24

The tower is 24 yards high.