Math Help - Statistics standard deviation

1. Statistics standard deviation

Can you help me with the following problem. I can work out that the variation is 2 standard deviations from the mean but how do I know that includes 95% of results?

A store manager determined the store's mean daily receipts as $550 with standard deviation of$200. On what proportion of days were the daily receipts between $150 and$950?

2. Re: Statistics standard deviation

Do you know the 68, 95, 99.7 rule?