A 3-foot pole casts a shadow of 9 feet.
How tall is a tree with a shadow of 12 feet? 

Let x represent the height of the tree.

The proportion is:

     TREE             POLE 
-------------- = --------------
SHADOW OF TREE   SHADOW OF POLE


          x     3
        ---- = ---
         12     9

Cross-multiply:

UPPER LEFT × LOWER RIGHT = LOWER LEFT × UPPER RIGHT

         9x = 36
         
          x = 4 feet

That's a very short tree!-- just a sapling! With a 
tree that short it would have been much easier for
them just to have measured the tree than to have 
measured the pole and the two shadows and then
done the above algebra!!!

Edwin