Step: 1
Temperature of Campbell′s bedroom = 33°C
Step: 2
Temperature of Campbell′s kitchen = 35°C
Step: 3
Temperature of Campbell′s refrigerator = - 13°C
Step: 4
Sum of the temperatures = 33°C + 35°C + (- 13°C)
Step: 5
= 33°C + 35°C - 13°C
[For any positive integer x and y, x + (- y) = x - y]
Step: 6
= 68°C - 13°C
Step: 7
= 55°C
Step: 8
So, the sum of the temperatures is 55°C.