Answer:
[tex]\frac{15}{30}[/tex] or [tex]\frac{1}{2}[/tex] is your answer
Step-by-step explanation:
First, add everything up,
7 + 5 + 3 + 15 = 30
This is what we have to so far
[tex]\frac{x}{30}[/tex]
Then, we put the number of red marbles in place of x.
[tex]\frac{15}{30}[/tex]
If you need the simplified version it is [tex]\frac{1}{2}[/tex]