You may solve elapsed time in terms of fraction, due to minutes involved.
3:45 am is also 03:45 hours = 3 [tex] \frac{45minutes}{60minutes} [/tex]
9:15 pm is also 21:15 hours = 21 [tex] \frac{15minutes}{60minutes} [/tex]
3:45 hours = 3[tex] \frac{3}{4} [/tex]
21:15 hours = 21[tex] \frac{1}{4} [/tex]
[tex] 20 \frac{5}{4} - 3 \frac{3}{4} = 17 \frac{2}{4} [/tex]
simplified:
[tex]17 \frac{2}{4} = 17 \frac{1}{2} [/tex]
or 17 hours and 30 minutes
