There's 23 questions. how many questions would you have to get wrong to get 62.5 %?

First, it is important to realize that the "62.5%" is the percentage of answers that you answered correctly. To change this percentage to the percent incorrect answers subtract the percentage from 100:

100-62.5= 37.5% of the answers are incorrect.

Now, multiply the total number of answers(23) by the percentage of incorrect answers(37.5% = = 0.375):

23×0.375= 8.625 answers. If you round up that is about 9 incorrect answers.
23-9= 14 correct ones 
(14/23)*100= 60.86% (this is the actual % you will get with 9 wrong answers, but it is closer to the percent in the problem than if we had rounded down to 8 questions.)
If we rounded down to 8 wrong answers we would get a 65.2%.
23- 8= 15 correct ones 
(15/23)*100= 65.2%