You turn 4/5 to 8/10 to get equal denominators and add the numerators together to get 16/10, or 1 6/10 which is reduced to 1 3/5.
If the quantity u have decreased with time so the rate is negative and vise versa
I'd say what you would do is:
You can either
1. Add 15 and 18 together and then divide by 2
OR
2. Since there is a 3 number difference between 15 an 18, just add 3 to 18
