Just how bad are you at maths?
You will need to be able to add, subtract, divide and multiply. You will likely be able to use a circular slide rule to do this (easy enough once you know how.) You should also be able to manipulate very simple formulas.
Example:
A x B = C
Now if you already know C and A but don't know B, how do you work out B?
C divided by A = B
A common use of the above is with distance and time equations, where speed x time = distance travelled.
Also, once you start flying a jet or high performance turbo prop you will need to be able to multiply by 3 in your head (to work ou a descent profile.) That's about as difficult as it gets in the real world of flying. It may be a little more tricky for the exams, but none of it involves particularly tough concepts.
Don't sweat it, it's not hard, though it can be a distraction trying to work things out while you are flying, luckily modern equipment does a lot of the work for you. But it's still good to do it roughly in your head just to check your equipment is giving good answers!