Answer:
75%
Step-by-step explanation:
First let's start by finding the total number of students at the assembly:
63+21=84
Then we find the percentage of the students that were first-graders.
63/84=9/12=3/4=75/100=75%
As further explanation to how this works:
We take the number of first graders over the total number of students so we can find the percentage. Then we simplify it down to 3/4. Since percentages are out of 100 we can change the fraction so it's something out of 100. We multiply the numerator and denominator by 25 (since 100/4=25) and we get 75/100 which is 75%
