I don't know how they do it in your class but I would do:

Then find the z-value for the the 90th percentile on a z-table (1.28)

So then we have:

So the upper limit for the central 80% is 628

Then

Then find the z-value for the the 10th percentile on a z-table (-1.28)

So the upper limit for the central 80% is 372

The scores for the central 80% are between 372 and 628.