You don't have to be a math wizard. If you can add, subtract, divide and multiply, you can follow most of what budget buildings do.
If you know how to calculate percentages, percentage increase and rates, you can see patterns that will help you explain the news. A little fuzzy on that? Here's help.
It also pays to know the tricks people play with numbers. The book "Proofiness," by Charles Seife, ought to be required reading for every journalist.