Last year the girls basketball had 8 5th graders and 7 6th graders. What was the ratio of 6th graders to 5th graders on the team

The ratio is 7:8

We simply list the number of sixth graders (7 of them), then write a colon, and then write the number of fifth graders (8 of them). After writing all this down, we would reduce the ratio but it cannot be reduced any further. Why not? Because there are no common factors (other than 1) between 7 and 8.

So that's why the answer is 7:8