Answer :
15/r = 5/3 (option 1)
Explanation:Ratio of black cards to red cards = 5: 3
If there are 15 black cards, let the number of red cards =r
The ratio of black cards to red cards becomes = 15: r
Since both expressions are prportional, we equate them together
5:3 = 15:r
Note: Ratio(:) means division
5/3 = 15/r
[tex]\begin{gathered} \frac{5}{3}=\frac{15}{r} \\ \text{rewritting:} \\ \frac{15}{r}\text{ = }\frac{\text{5}}{3}\text{ (option 1)} \end{gathered}[/tex]