How many questions can i miss on a 28 question test to get a 70%?

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%.

10.4 is the answer for you