I've set tests like this when recruiting at work - I'd expect it to be something similar to what you might have to do in the job. For uni PA/admin, we gave candidates some (fake) student figures for last year and this year, and asked them to draft a letter to the faculty dean about our unexpected increase in numbers - I was looking for accurate spelling and grammar, plus a bit of initiative, for example including something like 'please contact me to arrange a meeting with x' or 'as you'll see, this is a y% increase and we may need to consider the space and staffing implications' or similar.
We made the timing on the task pretty strict, so that it would be difficult to complete in the time given. We weren't necessarily expecting people to complete it, but to check how they'd prioritise the work/work under pressure.
Just remembered, we also gave them an 'in-tray task' - a list of things that they had to deal with on coming in to work, they had to order them from 1-10 to show what order they'd approach them (passing on info about someone ringing in sick etc first, catching up on filing last). Although we weren't bothered about the exact order, more that they'd given it some thought and it wasn't nonsense.
Hope this helps a bit. Do they ask for Word/Excel in the person spec? Maybe have a play around to make sure you are familiar with simple formulas if so - I've done a test myself where I had to create a simple budget - things like how to add a column of numbers and make the figure turn red if it's less than a certain amount (obvisouly no need to bother with this if you're unlikely to be using Excel in the job!).