Given the ratio of boys to girls in the class is 5:4, and there are 15 boys, we can set up the following proportion:

$\frac{\text{boys}}{\text{girls}} = \frac{5}{4}$

Let the number of girls be $x$. So, we can write the proportion as:

$\frac{15}{x} = \frac{5}{4}$

Now, solve for $x$ by cross-multiplying:

$15 \times 4 = 5 \times x$

$60 = 5x$

Now, divide both sides by 5:

$x = \frac{60}{5} = 12$

Therefore, there are 12 girls in the class.

Would you like further details on how this works? Or do you have any other questions?

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Related questions:

1. How would the number of girls change if the number of boys was 20?
2. What happens if the ratio of boys to girls is reversed (i.e., 4:5)?
3. If there were 10 more girls in the class, what would the new ratio be?
4. How can you solve for the total number of students if you know the ratio and one of the values?
5. What is the general method to solve word problems involving ratios?

Tip: Ratios are often easiest to solve using proportions, especially when you know one of the values. Always make sure to set the proportion up correctly before solving!