Write the problem as follows:
[tex]\text{21:15 - 08:30} [/tex]
Note that the maximum value for the minutes value is 59, as 60 minutes would convert into an hour.
Because we're subtracting 30 minutes from 15, convert an hour into 60 minutes for the first term, then subtract:
[tex]\text{20:75 - 08:30 = \boxed{12:45}}[/tex]
The resulting military time will be 1245.
