Try this book, it may explain it better: Engineering Maths (6th Ed. K. Stroud) pg. 1163
If I had my copy with my, I'd scan it in...
I could see how it could be done from a histogram, but would be an approximate solution. Stating your distribution as a normal distribution, you can achieve much more accurate answers... (As the histogram would have discrete steps, whereas a normal distribution has a continuos 'bell shaped' curve)
If you need to know anything else, just shout.
Dan