A groundhog sitting on its hind legs is 18 inches high and casts a shadow 3 feet, 9 inches long. If a nearby tree casts a shadow 21 feet, 3 inches long, how tall is the tree?

Let the height of the tree be x, then
18 inches / 3 feet 9 inches = x / 21 feet 3 inches
18 inches / 45 inches = x / 255 inches
45x = 18 x 255 = 4590
x = 4590 / 45 = 102

Therefore, the tree is 102 inches = 8 feet, 6 inches tall