1. **State the problem:** Find the ratio of boys to girls in the audience given that there are 240 girls out of 420 children. 2. **Identify the quantities:** - Total children = 420 - Girls = 240 - Boys = Total children - Girls = $420 - 240 = 180$ 3. **Set up the ratio:** The ratio of boys to girls is $\frac{\text{boys}}{\text{girls}} = \frac{180}{240}$. 4. **Simplify the ratio:** $$\frac{180}{240} = \frac{\cancel{180}^{\times 1}}{\cancel{240}^{\times \frac{4}{3}}} = \frac{3}{4}$$ 5. **Interpret the result:** The ratio of boys to girls is $3:4$. This means for every 3 boys, there are 4 girls in the audience.