To find the temperature at 10 PM, we calculate the decrease in temperature over time. The temperature decreases by 3.5°C each hour from 4 PM to 10 PM.

There are $10\, \text{PM} - 4\, \text{PM} = 6$ hours in this period.

The total decrease in temperature is:

$6 \times 3.5^\circ C = 21^\circ C$

Now, subtract the total decrease from the initial temperature:

$8^\circ C - 21^\circ C = -13^\circ C$

Thus, the temperature at 10 PM was $-13^\circ C$.

Would you like more details or have any questions?

Here are some related questions to expand on this:

1. How would the answer change if the temperature decreased by 2°C per hour instead?
2. What if the temperature decreased at a variable rate over the 6 hours?
3. How could we represent this situation as a linear equation?
4. What is the significance of negative temperatures in Celsius?
5. How does this compare to temperature changes in Fahrenheit?

Tip: Always note the rate of change