Id it possible to create calculated fields in Ireport?

I have an sql statement that returns name num1 num2 num3

What I want is to be able to add num1 to num2 and divide it by num3

Then do the same in the total

I would have thought this should be really easy but I can't see how to do it and I have bought the 'definitive guide uo ireport'

Any help would be much appreciated