Final answer:
The temperature change per hour is calculated by dividing the total temperature change by the total time. This gives us -1 3/5 degrees per hour, indicating a decrease in temperature, which is answer (b).
Step-by-step explanation:
The student's question relates to the rate of change in temperature over time. The temperature fell from 0 to 6 3/5 degrees below zero in 21/5 hours. To find the temperature change per hour, we can use the formula: rate = change in value / change in time.
Total change in temperature = 6 3/5 degrees = 33/5 degrees (since 6 3/5 = 6 + 3/5 = 30/5 + 3/5 = 33/5)
Total time = 21/5 hours
Temperature change per hour = (33/5) / (21/5) = 33/5 × 5/21 = 33/21 = 1 12/21, which simplifies to 1 4/7 or 1 4/7 × 5/5 = 7/7 + 20/7 = 27/7 / 5 = 27/35
When we simplify 27/35, we get 3/5, so the rate of change in temperature per hour is -3/5 because the temperature is dropping, and hence this is a negative change.
The correct answer is therefore -1 3/5 degrees per hour, which corresponds to option (b).