trouble with 'double' in Visual C++ 5.0

I am creating a service report and the users have to enter in the time they started
   to the time they finished and I want to calculate the amount of hours they have
   worked.  The method I am using seems to work the odd time but I am having
   trouble getting it to work all the time.  I am using double variables and I am not
   sure if I should be using them. I am also changing the time into 24 hr clock if the
   starttime is larger than the endtime.
   eg. starttime  11.00  endtime 2.00->14.00
   This works all the time but it is when you subtract the time such as 1.15 to 2.00,
   you get 0. 45  but I want it to be .75 because you have worked 3/4 of an hour.  I
   am only doing it in 15 minute intervals so I didn't think it woul be that difficult.  This
   is how I am doin it.
   if (m_hourswork==0.45) //45 minutes worked
       m_hourswork=0.75;
   if m_hourswork==0.15)  //15 minutes worked
      m_hourswork=0.25)
   I do this for all 15 min intervals and every hour upto 10 hours because no one will
   work more than 10.  The method I am using seems to work from time to time so I
   think it may be the 'double' varibles I am using.

   Thank You for the help!!
   Greatly Needed