Answer: He used 5 3/4 kg in all. There was 9 1/4 kg left.
Explanation:
Add 3 1/2 and 2 1/4 to find how much he used. The common denominator is 4 so convert 3 1/2 to 3 2/4. Then add. The answer will be 5 3/4.
Subtract 5 3/4 from 15 to get how much is left. The answer will be 9 1/4.