If you turn 20% into a decimal the it will become 20 itself then you can do 20x5 to get 100, then you can use the 9 and the 5 and multiply it together then your answer is 45.
What I did is I multiplied 9 by 5 and got 45. I multiplied 9 by 5 because the question stated that 9 students are 20%. 20% times 5 would be 100%, which would be the total number of students.