Explain the three structures of subtraction word problems with examples.

Here are the explanations for the different subtraction problem structures:

a) Result Unknown
This structure is used when you know the starting amount and the amount being subtracted, and you need to find the final amount or the difference.
Example: You have 5 apples and eat 2. How many apples are left?

$$5 - 2 = \boxed{x}$$ $$x = \boxed{3}$$

b) Change Unknown
This structure is used when you know the starting amount and the final amount, but you need to find out how much was subtracted.
Example: You had 5 apples, and now you have 3. How many apples did you eat?

$$5 - \boxed{x} = 3$$ $$x = 5 - 3$$ $$x = \boxed{2}$$

c) Initial Unknown
This structure is used when you know the amount that was subtracted and the final amount, but you need to find the original starting amount.
Example: You ate 2 apples, and now you have 3 apples left. How many apples did you have to begin with?

$$\boxed{x} - 2 = 3$$ $$x = 3 + 2$$ $$x = \boxed{5}$$