I'm still unclear on how to complete this type of problem.

This is what I've been taught to do, and I'm getting the wrong answer. Please help.

1.645 times 31.5 square root N.

1.645 times 31.5= 51.8175.

51.8175 squared= 2685.05. I'm way off please help.

You want to obtain a sample to estimate a population mean. Based on previous evidence, you believe the population standard deviation is approximately =31.5. You would like to be 90% confident that your estimate is within 2 of the true population mean. How large of a sample size is required? As in the reading, in your calculations: --Use z = 1.645 for a 90% confidence interval --Use z = 2 for a 95% confidence interval --Use z = 2.576 for a 99% confidence interval.