Yes, I think they usually allow a calculator and scrap paper, though I don't find that always helps!

I did the most horrendous one you can imagine for Eversheds earlier in the year - I only got through half the questions and I wasn't all that sure about the answers for those, and I always liked Maths! There are lots of practice numerical tests on the web, though - definitely try the SHL ones. I also remember finding a website that went through things like percentages, fractions, etc in quite a straightforward way, specifically for people doing tests of this type.