Final answer:
The largest number of groups that could be formed is 15, with 6 sixth-graders and 5 seventh-graders in each group.
Step-by-step explanation:
To find the largest number of groups that can be formed with the same combination of sixth- and seventh-grade students, we need to find the greatest common divisor (GCD) of the number of sixth-graders and seventh-graders. The GCD of 90 and 75 is 15. Therefore, the largest number of groups that could be formed is 15.
Each group will have an equal number of sixth- and seventh-graders. To find the number of students of each grade level in each group, we divide the total number of each grade by the number of groups. The number of sixth-graders in each group is 90 divided by 15, which is 6. The number of seventh-graders in each group is 75 divided by 15, which is 5.