Answer:
50 7th grade and 40 8th grade
Step-by-step explanation:
1. add to get total number of students, 1440
2. find percentage of total that are 7th grade students and 8th grade student:
640/1440 = ~45% (are 7th grade)
800/1440 = ~55% (are 8th grade)
3. 55% of 90 is 50, and 45% of 90 is 40