1. **State the problem:** Explain why the equation uses $2g$ instead of just $g$ when solving for the number of girls. 2. **Recall the equation:** The total number of boys and girls is given by: $$g + (g + 35) = 137$$ 3. **Combine like terms:** Here, $g$ represents the number of girls, and $g + 35$ represents the number of boys. Adding these together: $$g + g + 35 = 137$$ 4. **Simplify:** Combine the $g$ terms: $$2g + 35 = 137$$ 5. **Explanation:** We use $2g$ because there are two groups involving $g$: the girls ($g$) and the boys ($g + 35$), which includes another $g$. So, adding them together gives $2g$. 6. **Summary:** The $2g$ comes from adding the number of girls and boys (expressed in terms of $g$), not just the number of girls alone.