Final answer:
Ann needs 3.5 cups of sugar to serve 70 people with this recipe.
Step-by-step explanation:
To find out how many cups of sugar Ann needs to serve 70 people, we need to set up a proportion using the information given. The recipe serves 20 people with 1/2 cup of sugar. So, the proportion will be:
20 people / 1/2 cup = 70 people / x cups
Next, we can cross-multiply and solve for x:
20 * x = 1/2 * 70
x = (1/2 * 70) / 20
x = 3.5 cups
Therefore, Ann needs 3.5 cups of sugar to serve 70 people with this recipe.