With the vocabulary challenge, you can earn 150 kronur per round. Meaning there is a total of 600 kronur you can get in any four-round challenge.
You need 2050 kronur, to buy things at Soren's. That means, in the fastest possible route, you do the vocabulary challenge four times in a row.
If you miss two answers, you miss out on 90 kronur. So...when you play the challenge four times in a row, you can miss a grand total of seven answers, while still getting more than 2050 kronur. That's quite a tall order.