A recipe calls for 2 1/2 cups of flour. If you have already used 3/4 cups, how much more flour do you need to complete the recipe?
To find out how much more flour is needed, you subtract the amount of flour already used from the total amount needed.
2 1/2 cups - 3/4 cups = 2 cups + 1/2 cup - 3/4 cups
To subtract fractions, you need a common denominator. In this case, the common denominator is 4.
2 cups = 8/4 cups
1/2 cup = 2/4 cups
8/4 cups + 2/4 cups - 3/4 cups = 10/4 cups - 3/4 cups
To subtract fractions with the same denominator, you subtract the numerators and keep the common denominator.
10/4 cups - 3/4 cups = (10 - 3)/4 cups = 7/4 cups
Therefore, you still need 7/4 cups of flour to complete the recipe.
Note: 7/4 cups can also be written as 1 3/4 cups.