How about giving them a real task that your payroll team carry out - just anonymise the names and maybe change the amounts. Eg. calculating maternity pay or sickness deductions?

You can give a brief outline of the policy, the background (ie. length of service, sickness absence dates) and ask how much should be paid/deducted.

In reply to Anna:

I like it thank you!

In reply to Samantha:

Hello there! Consider using HireNest's pre-employment assessment services if you want to determine whether an applicant can manage simple payroll scenarios. Here's the link to their website: https://hirenest.com/. They offer many hiring tests, so I am sure everyone can find the one that fit their needs the best.

In reply to Evelina:

Or we do a general numeracy test as one of our assessments - www.simusuite.com they are all pay as you go and you can choose which you use.
There isnt a specific payroll one - but having been the recipient of payroll errors in the past I was amazed at how many payroll admins don't seem to have any proficiency with mental maths so can't just look at a number and know it is "wrong" and so to redo the calcs.