ord problem...how do I write this out in Algebra (ratio and proportion) form?

I know the answer is 9, however I do not know how to show the formula I used to arrive at the answer!:

A cake recipe serves 4 people, and calls for 1.5 cups of flour and 2.5 cups of sugar. In the cupboard there is 3.5 cups of flour, and 10.25 cups of sugar. Using the recipe to make the largest cake possible with the all ingredients, how many people will it serve?

I know the problem lies with the flour I eed to know lso te ratio of extra sugar to the extra .5 cup of flour! I appreciate your help in showing me the steps, and how to write this out so that it makes sense, in algebra terms!!!