Since you didn't give me a total # of questions on the test, I can't give you a direct answer....
But I can walk you through.
In math, "of", means you have to multiply.
80%= .8 This is bc 100% is = 1.00.
() mean times or multiplied by
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So you would do .8(how ever many total questions were on the test)
Whatever # you get is the answer.
Hope I could help!!!