Answer:
15 students suggested a rainbow-themed party.
25 students did not suggest a rainbow-themed party.
Explanation:
40 students
3 out of every 8 students suggested rainbow them
40/8 = 5
5 groups = 40 students
1 group = 8 students
3 × 5 (amount of groups) = 15
15 students suggested a rainbow-themed party.
40 - 15 = 25
25 students did not suggest a rainbow-themed party.