[-½, ¾]; use either "Substitution" or "Elimination" to do this, and you'll see that this answer is correct.
1/4 of 12 simply means that if there are 4 equal quarters in 12, how many would be in each of those quarters, or 1/4*12 or 12/4. 12 divided by 4 means that the 12 is on the top and 4 is on the bottom, meaning 12/4.
If you would like to know how much did the bag of grapefruit weigh, you can calculate this using the following steps:
7 1/2 pounds - 1 5/8 pounds of apples - 2 3/16 pounds of bananas = 7 1/2 - 1 5/8 - 2 3/16 = 15/2 - 13/8 - 35/16 = 120/16 - 26/16 - 35/16 = 59/16 = 3 11/16 pounds of grapefruit
The correct result would be 3 11/16 pounds of grapefruit.