Answer: 18 feet
Step-by-step explanation:
If you draw out the figure, you should have 2 similar right triangles.
The similar right triangles allow us to form a proportion in the format
A/B = C/D
where
- A = tree's height
- B = shadow length of the tree
- C = man's height
- D = shadow length of the man
In this case
- A = 9
- B = 12
- C = unknown for now
- D = 24
Let's solve for C.
A/B = C/D
9/12 = C/24
9*24 = 12*C ... cross multiplying
C = 9*24/12
C = 9*2
C = 18
Of course this is very unrealistic to be this tall. It appears your teacher wasn't too careful in selecting numbers.