Since we are asked to find the rate of change from Monday to Wednesday, we are dividing by 2, since Monday is day 1 and Wednesday is day 3. 3 - 1 = 2.
1800 + 500 = 2300 calories consumed Tuesday.
2300 - 100 = 2200 calories Wednesday.
(2200 - 1800)/2 = 400/2 = 200 calories