25% of the 6th graders were on the choir trip.
Since the question states that 40 6th grade students went on a choir trip and 120 6th grade students remained at school, you must add together 40 and 120 to get the total number of 6th grade students. By doing so, you'll get 160 6th graders.
Then, you divide 40 by 160. This will give you a decimal of 0.25, which you multiply by 100 to make it a percent. 0.25 times 100 = 25%. Thus meaning 25% is the answer.
I hope this helps!
