If a recipe uses 3 ½ cups of flour for ½ the recipe, how much flour is needed for the entire recipe?

3 1/2 + 1/2.

If you make the mixed number an improper fraction by multiplying the whole to the denominator and then add the numerator. This would be your new numerator, but then keep the original denominator. But since this is working with halves you could easily do this

1/2+1/2 = 1 3+1 = 4 cups of flour for the whole recipe

It needs 7cups of flour