Problem Analysis and Solution:

Question (a):

• Initial temperature: $10^\circ C$ at 9 a.m.

• Temperature dropped by $15^\circ C$ at midnight.

  Calculation: $\text{Temperature at midnight} = 10^\circ C - 15^\circ C = -5^\circ C$

  Answer (a): The temperature at midnight was $-5^\circ C$.

Question (b):

• Temperature at 9 a.m.: $10^\circ C$

• Temperature at 9 p.m.: $-1^\circ C$

  Calculation: $\text{Temperature drop} = 10^\circ C - (-1^\circ C) = 10^\circ C + 1^\circ C = 11^\circ C$

  Answer (b): The temperature dropped by $11^\circ C$ between 9 a.m. and 9 p.m.

Would you like me to explain this in more detail or clarify any step?

Related Questions:

1. If the temperature dropped another $3^\circ C$ after midnight, what would the new temperature be?
2. What is the total change in temperature from 9 a.m. to midnight?
3. How would the temperature change if it rose $7^\circ C$ from the midnight value?
4. If the temperature at 6 a.m. was $5^\circ C$, what was the total change from 6 a.m. to midnight?
5. Can you express the temperature changes in absolute values?

Tip:

When calculating temperature differences, pay close attention to signs (positive or negative) to avoid errors!