The idea is almost the same as before:
The probability is again # keyboard players of seventh grade /#total of students of seventh grade = 9/35.
It is the answer because we can't simplify 9/35 (there are no common divisors distinct from 1)
Additional comment: The dates on the eighth-grade students shouldn't be considered because we are choosing a student of seventh-grade, so the eighth-grade students don't "affect" the probability.