Answer:
Micheal is 8, and Tom is 12 years old.
Explanation:
To solve this, we can set each of their ages as a variable. Let's say Michael's age is x. 
We know Tom is 4 years older than Michael, so Tom's age is x+4. 
We also know that their combined age is 20, so if we add both of their ages, we should get 20. 
Solving;

 So Michael's age is 8, and Tom is 12.