Question

All of the students who took chemistry at Groveton High School last year were juniors or seniors, as shown in the table below. If 80% of the students who took chemistry passed, then what fraction of the students in the combined junior and senior classes took chemistry?

Class Total # of Students in Class # of Students who passed
Junior 80 42
Senior 70 30

Answer 1

The correct answer is 3/5, but I don't know how. Thank you!

Answer 2

72 students passed so
72/0.8 = 90
90/150 can be simplified to 3/5

Check my thinking