Coded-to-spec, efficient, simple and accurate.
Nice use of the named constant const double NumDays::perday = 8.0; It's a C++ naming convention to make named constants all CAPS as a signal to the reader it is non-changing, when they see it used in the code. So perday would be PERDAY.
Nice job. Clean, efficient code that compiles and runs great. You created the different classes which interacted so well