Interpreting the wording of questions about correcting errors/double entries

Goran has made the following errors.
1. a purchase of $1000 was entered in the purchase day book as $100
2. a purchase of $500 was correctly recorded in the payables control account and was debited to the vehicles account
How is Goran’s trial balance affected by these errors?

This isn’t as much of a wording question and more to do with day books- i thought 1 just meant that the entry was
Dr purchases 100
Cr AP 1000
but since the answer is: debits will equal credits then that can’t be the case. both sides were entered as 100
how do I know if the error is on both Dr and Cr or just one side? My initial thought is that only the Dr purchase was incorrect. So i often get these questions wrong.