How do you find the mean from a box-and-whisker plot?

I had thought that the box-and-whisker plot does not show the mean, but, on my homework, it is telling me to find the mean. It only gives me three box-and-whisker plots (not a list of numbers), and is telling me to find the mean for each one. Can anyone tell me how this is done? Please, it will be a great help!