Final answer:
David needs to buy 9 roses from either florist to be charged the same total amount, which includes the price per rose and the delivery fee for each florist.
Step-by-step explanation:
We have two florists: Flowers Florist, which charges $2 per rose and a $10 delivery fee, and Rubies Roses, which charges $1 per rose and a $19 delivery fee. Let's denote the number of roses David buys as x. The total cost for Flowers Florist will be 2x + 10, and for Rubies Roses will be x + 19. We want to find the value of x where both costs are equal: 2x + 10 = x + 19. To solve, we subtract x from both sides: 2x - x + 10 = 19, x + 10 = 19. Now, subtract 10 from both sides: x = 19 - 10, x = 9. So, David needs to buy 9 roses for both florists to charge him the same amount.