Chapter 1 - Foundations for Algebra - 1-6 Multiplying and Dividing Real Numbers - Practice and Problem-Solving Exercises - Page 42: 38

-30 ft/minute

In order to figure out the average speed, we divide the distance that the scuba diver falls by the time it takes him to fall that far. Thus, we divide -90 by 3. A negative divided by a positive or a positive divided by a negative is always a negative. Thus, it follows that: $-90\div 3=-30$