When you arrive at work you need to record in the log book the exact time you get to work, and then exactly what time you leave. when you worked on wednesday you started work at 9.30am and left work at 6.05pm. what was the total time you worked for?

Counting forward, there are 9 hours between 9:30 am and 6:30 pm.
Since you left at 6:05 pm, which is 25 minutes from 6:30 pm, subtract 25 minutes from 9 hours:
8 hours 35 minutes

You could also convert the time to 24-hour time (18:05 instead of 6:50 pm) and change it from hours and minutes to just hours:
9:30 am → 9 + 1/2
6:05 pm → 18 + 1/12
and subtract the two, then convert back into hours and minutes