Hey Mike, on question #3 page 189 I dont understand how to make the equation with the problem and the explanation didnt help me.

Remember, averages questions are usually about sums. If the average of the first days is 87 degrees, then the sum of those days must be . The next day it was 93 degrees, so the sum of the first days is .

So far, so good, right?

Now, we *also* happen to know that the average for those days was 89 degrees. Just like we did before, we can multiply the average by the number of data points to find the sum: .

Here’s the thing. Both and represent the same sum of the temperatures over the days. So you can set them equal and solve.

Does that help?