Ashadow 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.