The other replies in the thread have covered the practical answer to your question (which is: there's no way to say, because the GMAT doesn't really count how many you get right/wrong.)
However, if anyone is curious, I have a little data from score reports.
- You'd probably get about a 720 if you got a 40 on Verbal and a 48 on Quant, which is a pretty reasonable breakdown for people scoring in this range.
- In my score reports, people scoring around a 40 on Verbal got anywhere from 67% to 80% of the questions right. (However, getting that many questions right doesn't guarantee you a 40. For instance, one student got 74% of the Verbal questions right overall and scored a 28.)
- People scoring around a 48 on Quant got anywhere from 64% to 82% of the questions right. (Again, getting that many questions right doesn't guarantee you a 48. For instance, one student got 75% of the Quant questions right and scored a 38.)
The point of this isn't to give you a target to aim for, because
getting a certain number of questions right does not guarantee a certain score. In fact, it doesn't even make it too much more likely that you'll get that score. The point is to drive home what everyone already said - that there's not a very strong correlation, and you're better off focusing on performing consistently and getting through the whole test section within the time limit.