Return sum info from specific column on separate worksheet

I need help on returning the sum value of specific time entered by different individuals from another workbook. Attached is a small snap shot of the workbook. I have an employee list that may be up to 20 individuals and I need to returrn the total number of hours they collectively entered. Basically I am looking for a furmula that will return the number of hours in column L for any number of different employees in column B