Set u a ratio of height over shadow:
Lance = 5 feet tall / 8 foot shadow
Tree = X feet tall / 32 foot shadow.
Set the ratios equal to each other and solve for X:
5/8 = x/32
Cross multiply:
8x = 160
Divide both sides by 8:
x = 160/8
x = 20
The tree is 20 feet tall.