Answer:
Here we have a recipe that works for 10 serves.
Then, if we want to make 30 servings, we need to use that recipe 3 times. (because x*10 = 30, then x = 30/10 = 3)
This would be equivalent to multiply each ingredient by 3.
for 30 cupcakes we need:
3 cups of vegetable oil.
3 cups of all-purpose flour
(because for each one of these ingredients we needed only 1 cup to make 10 cupcakes).
for 10 cupcakes we need 1 teaspoon of vanilla.
for 30 cupcakes we need 3 teaspoons of vanilla, the difference is 3 - 1 = 2.
So the difference is 2 teaspoons.
About which quantities we should round are those where we may have mixed numbers or big numbers.
For example, we have 4 tablespoons of sour cream for 10 servings, then for 30 servings we should have:
3*4 = 12 tablespoons.
We could round this down to 10 tablespoons.