It is 8:50, 10 minutes before 9:00.
From 9:00 to 10:00 is 60 minutes, or one hour.
10:00 to 10:30 is 30 minutes.
So, take all of the minutes you figured out and add them.
10+60+30=100
One hour is 60 minutes, so if we subtract that from 100 minutes (100-60) we are left with 40 minutes.
Of course, this is an exact timing. If she wants to arrive 15 minutes earlier,
100-15=85
85-60=25
So, then she could put a timer for 1 hr 25 mins.
Therefore, Sally must put a timer for 1 hour and 40 minutes, if she wants the exact time of the class.
Hope this helps.