Finding Expected Value and Standard deviation...Help please :(

The question has a table that contain shows the approximate number of males of Hispanic origin employed in the U.S. in 2005 broken down by age group

Age: 15-24.9 25-54.9 55-64.9

Employment (thousands):16,000 13,000 1,600

It then asks that using the midpoints of the given measurement classes, compute the expected value and the standard deviation of the age X of a male Hispanic worker in the U.S...

The back of the book tells me that the Mean is 30.2 years and a standard deviation of 11.78 years but I have absolutely NO idea how these were obtained. The chapter that covers this question is a chapter about Measures of Dispersion if that helps at all. Thank you for all your help.