Assume that an A is a 90% (getting at least 9 questions out of 10 right).
The probability that exactly 9 questions are right is 10 (choose one question to get wrong) * (1/4)^9 (1/4 chance of getting each question right) * (3/4) (chance of getting the wrong question wrong) = 10∗3∗(1/4)10 .
The probability that all 10 questions are right is (1/4)10 (1/4 chance of getting each question right).
The total probability of getting an A is (10∗3+1)(1/4)10=31410, or about 0.002956%.
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