We can change the 3 hours and 20 minutes to "minutes" instead of hours:
1 hour is 60 minutes. So 3 hours is 60 * 3 = 180 minutes.
Add the 20 : 180 + 20 = 200 minutes for one costume.
Multiply by 6 for 6 costumes: 200 * 6 = 1200.
So 1200 minutes or 1200/60 = 20 hours to make 6 costumes
