One morning I had a math test and I did very badly on it.
Oh no, that is a shame. What caused you to do so poorly?
I don't know how to multiply two digit numbers or something like that
This is precisely why you need to study for this stuff!  Passing tests is important to your future and they're well worth a little studying for.  I hope you do better next time!
I'll just pretend it doesn't matter and see what happens.
How about you try studying for just a bit next time and see how that helps you. You may find that a little effort will go a long way toward a better grade.