Std Dev Bounds

I have a question that I think should be straight forward but the automated tool keeps telling me I'm wrong and I think I just need a little help. Here is the formal question:

"Given the company’s performance record and based on the empirical rule of normal distribution (also known as the 68%-95%-99.7% rule), what would be the lower bound of the range of sales values that contains 68% of the monthly sales?"

Here is the key data from the sheet, which have all be confirmed as correct:

Median 8124.23
Mean 8235.78
Std Dev 780.93

I've tried taking the mean and subtracting 1 Std Dev giving me 7343.30 which was wrong. I tried using a function in excel Norm.Inv with a .34 probability which gave me
7913.68 which was also wrong. If you need the full sheet I can attach it. Thanks in advance.


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I don't know how you did the subtraction but the mean you provided minus the standard deviation you provided doesn't result in what you said you got.