Sara was thinking of a number. Sara subtracts 9 from it and gets an answer of 0.4. What was the original number?

Can someone explain the elimination method to me for this problem?

Describe in detail how you would construct a 95% confidence interval for a set of 30 data points whose mean is 20 and population standard deviation is 3. be sure to show that you know the formula and how to plug into it. also, clearly state the margin of error.

Which of the following expressions are equivalent ?