To find the ratio of the number of students who have chocolate ice cream to the number of students who have vanilla ice cream, follow these steps:

1. Number of students with chocolate ice cream: 4
2. Number of students with vanilla ice cream: 9

The ratio is given by:

$\text{Ratio} = \frac{\text{Number of students with chocolate}}{\text{Number of students with vanilla}} = \frac{4}{9}$

So, the ratio of students who have chocolate ice cream to those who have vanilla ice cream is 4:9.

Would you like more details or have any other questions?

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Here are some related questions:

1. What is the ratio of vanilla to chocolate ice cream?
2. If 3 more students get vanilla ice cream, what will the new ratio of chocolate to vanilla be?
3. How would the ratio change if 2 students with vanilla ice cream switch to chocolate?
4. What percentage of the total students have chocolate ice cream?
5. What is the simplified ratio if both the numbers were doubled?
6. How does changing the number of students with chocolate affect the ratio?
7. If all students received ice cream, what fraction of them have chocolate?
8. How would you express the ratio of chocolate to total students?

Tip: Ratios can be simplified like fractions; always look to simplify them when possible.