For the PPL:
You should know multiplication tables by heart;
You should be able to add, subtract and do simple divisions in your head; and
You should have an understanding of ratios and proportions.
For the CPL, replace 'should' with 'must' and add:
A basic grasp of square root, sin and cos functions.
As Piltdown Man said: Arithmetic is regularly used in flying.
You don't have to be any kind of expert in maths, but you will have a few problems if you are not comfortable dealing with numbers.
I did instruct someone up to PPL level who was dyslexic and I didn't find out until the navigation cross-country stage. He did finish the course but struggled a lot with some of the written exams.
Good luck!