Final answer:
To express Paul's age mathematically compared to Bill's age, we use (x+a) years, where x is Bill's age and a is the number of years by which Paul is older than Bill.
Step-by-step explanation:
The problem states that Paul is older than Bill and assigns the age of Bill as x years, where x is a natural number. We can represent Paul's age in relation to Bill's age using arithmetic operations. Since Paul is older than Bill, we need to add a certain number to Bill's age to get Paul's age. Therefore, the correct representation of Paul's age would be a quantity greater than x, so it can be expressed as (x+a) years, where a is a positive natural number that represents how many more years Paul is older than Bill.