Interpreting Standard Deviation

This is the problem: The output from a statistical computer program indicates that the mean and standard deviation of a data set consisting of 200 measurements are $1,500 and $300. What can be said about the number of measurements between $900 and $2,100; between $600 and $2,400?

I know the answers are "at least 3/4" for $900-$2,100 and "at least 8/9" for $600-$2,400. I have no idea how to get there. I've been trying different things and looking at the mound but I just need an explanation. Please and thank you.