18 questions
In order to get an A on the exam, Alyssa needs a ratio of 90 : 100. Looking at the solutions in part (a), 100 is 5 times 20 so to see how many questions Alyssa needs to answer correctly out of 20 we can divide 90 by 5. Since 90 \div 5 = 18, Alyssa needs to answer 18 questions correctly.
