First find the total length of the appointments: 2 appointments * 3 hours/appointment = <<2*3=6>>6 hours
Then subtract that time from the length of Andrew's workday to find how many hours he spends stamping permits: 8 hours - 6 hours = <<8-6=2>>2 hours
Then multiply that number by the number of permits he can stamp per day to find how many permits he stamps: 50 permits/hour * 2 hours = <<50*2=100>>100 permits
#### 100