For the first question:
-8.3 - (-5.4)
This is -8.3 minus negative 5.4
The two negatives equal a positive, so he <em>should have </em>replaced them with a + sign:
-8.3 + 5.4
Instead, he subtracted 5.4 from -8.3 and got A.
For the second question, find a common denominator so that you can add the fractions together. 8 and 3 both fit into 24, so use that as your denominator. To get from the denominator 8 to the denominator 24, you multiply by 3. So multiply the numerator (7) by 3 as well. So that fraction becomes 21/24
To get from denominator 3 to 24, you multiply by 8. So do the same thing to the top. You then have 16/24. The change in the amount of water is 16/24 cups. That can be simplified to 2/3
