Subtracting Hours And Minutes

I am trying to calculate employees hours of work from information provided to myself.

Each employee is contracted to work 40 hours per week. The information provided to myself is in the format - Employee has worked 39.30 This is in fact 39 hrs 30 minutes. Obviously when I subtract 39.30 from 40.00 I get 0.7 and not 30 minutes. I tried formatting the cell containing these figures to hh:mm but it comes out at 15.30 & 15.00 respectively because it is over 24 hrs. How do I convert the digital number (0.7) to minutes I.e. 30 minutes.

I have tried to convert the 40 hrs into minutes but because the hours are greater than 24 then I struggle.