Problems should use the Courier font. Why? Every Courier font character uses the same amount of space. A comma is the same width as the number 5. This means that all of the numbers line up perfectly for carrying during addition and bringing the zero down in division.
Some free math worksheet sites allow you to choose criteria, click a button, and generate a web page of problems. These types of web sites allow for worksheets that produce random digits in numbers. You should find more robust problems sets on web_generated math worksheets. How can web sites give away math worksheets? Do free math worksheet web sites make money? Yes and no. Let's just say it is very important that you visit and support your favorite free math worksheets web site!