Final answer:
The temperature when sweater sales are at $1,200 is -3 degrees Celsius.
Step-by-step explanation:
To find the temperature when sweater sales are at $1,200, we need to solve the given equation for t, which represents the temperature. The equation is s = 900 - 100t, where s represents the sales in dollars. We are given that the sales are $1,200. Therefore, we can substitute this value into the equation and solve for t: 1200 = 900 - 100t. By rearranging the equation, we get -100t = 1200 - 900, which simplifies to -100t = 300. Dividing both sides of the equation by -100, we find that t = -3.