I've just been wondering what mathematical skills I would need to become a software tester.
Kairu I wonder what has stimulated your question? Why do you think that you need mathematical skills?
I'd say the most important things in no particular order are:
When looking at performance, my own specialist field, a knowledge of basic statistics is good. Still amazes me the number of people who set or monitor Service Level Agreements (SLAs) who do not know the difference between a percentage and a percentile.
When I started out many years ago I arrived at IT because I studied mathematics. I'm sure many ITers liked mathematics more than say literature. But I do not think it is particularly relevant.
As Nigel has already observed, software testing is a field where the most important skills tend to be difficult to quantify (good communication skills, for example).
In my experience, unless you're testing something that involves heavy mathematical lifting, you don't need more than high school level mathematical skills.
A knowledge of basic statistics is always good. So is being able to dig into a database and spot-check calculations (although my experience is that most of the challenge there is refining the data-set so that the calculations can be performed by a human in a reasonable time-frame).
Here's some of the mathematical skills I've used in my testing career:
The logical skills (which can be classed as mathematical or not, depending on who's doing the classifications) I use include:
"Soft" skills I use:
This is all a long way from a complete list, but it should be enough to give you an idea of where mathematical skills fit in the overall testing picture.
Statistics is probably a must in order to understand testing, for example the Birthday Paradox that can be used to reduce the number of tests, analyzing numerical results of tests and performance tests or planning stop conditions of test runs.
Some basic (or even more) Algebra and Combinatorics are also important for solving day to ay algorithmic problems.
Unless you test mathematical software, the most you would need is Algebra. There are other skills you will need, but Algebra should cover the math part.