To find the profit \( P \) for Lena, we can set up the equation based on her costs and earnings. The total profit \( P \) can be calculated using the formula:
\[ P = 5n - 200 \]
where:
- \( P \) is the profit,
- \( n \) is the number of pairs of earrings sold,
- \( 5n \) is the total money made from selling \( n \) pairs of earrings,
- \( 200 \) is the cost of renting the booth.
Now, to find how many pairs of earrings (\( n \)) Lena must sell to earn a profit of $450, we set \( P \) to 450:
\[ 450 = 5n - 200 \]
Now, let's solve for \( n \):
- Add 200 to both sides:
\[ 450 + 200 = 5n \]
\[ 650 = 5n \]
- Divide both sides by 5:
\[ n = \frac{650}{5} \]
\[ n = 130 \]
Lena must sell 130 pairs of earrings to earn a profit of $450.