Estimating the mean using confidence levels

A company is interested in estimating the mean number of sick days taken by its employees. The firm's statistician selects 100 files at random and ntoes the number of sick days taken by each employee. The mean was 12.2 days and the standard deviation was 10 days. Estimate the mean with 90% confidence and interpret.

I got (11.662, 12.738) with an E of .538 after doing the formula for E

My professor wrote down the answer of (10.555,13.845), which corresponds with an E of 1.645 if you do the subtractions.

1.645 is also the Critical Value for a 90% confidence level, what in the world is going on here?