Question 748632
80% of 70, assuming all the questions are weighted the same, is 56.  (.80)(70)=56

Thus, the minimum amount you could get right is 56 out of 70 for an 80%. 

70-56=14

Therefore, you can miss 14 questions at most.

Alternatively, if you want 80% right, you could have at most 20% wrong.  
20% of 70 is (.20)(70)=14