In this scenario, I'm assuming each question has the value of one point. Begin by taking 70% of 28 to represent the exact score you need:
0.7(28) = 19.6
You need to get 19.6 points for a 70%. To calculate the points missed, subtract this from the total points on the test:
28 - 19.6 = 8.4
You can only miss 8.4 points on the test, or no more than 8 questions, to get a grade of 70%.