On Sunday, Sheldon bought 4 ½ kg of plant food. He used 1 ⅔ kg on his strawberry plants and ¼ kg for his tomato plants. How many kilograms of plant food did Sheldon have left?

1 2/3 + 1/4 = 5/3 + 1/4 = 20/12 + 3/12 = 23/12

so
4 1/2 - 23/12 = 9/2 - 23/12 = 54/12 - 23/12 = 31/12 = 2 7/12