Ok, this may be confusing, but this is how I would figure it out. First you determine that since the building is 40 ft tall, and the shadow, 16 ft, is 40% of the size of the building itself. (4 goes into 40 and 16 evenly, so 16/4=4 40/4=10 So 4/10=40%) So now we need to figure how tall the tree is. If 18 is only 40% of the size of the tree, then we do this: 18/4=4.5 (Because if the shadow=40%, we can reduce this and find out what 10% is equal to of the height of the tree, to make the process simpler) So now we have 10% of the tree, which is 4.5, and all we do is multiply(4.5) times 10 to equal 100% of the tree's height. 4.5x10=45. The tree is 45 ft tall.
