Because one batch of cookies uses up 1.5 cups of flour, we can write this as:
batch of cookies = 1.5 cups of flour
Because she made 3 batches we can say:
batch of cookies = 1.5 cup of flour * 3 (because she has made three)
This equals to 3 batches of cookies using up 4.5 cups of flour.
Because at the beginning she had 5 cups and she has used 4.5 cups of flour for making the batches of cookies, she is left with 0.5 cups of flour:
5 cups at the start - 4.5 cups used for cooking = 0.5 cups in the end.
