Divide 2 hours into tenths of an hour and we get 20 tenths.
Each play is 3 tenths and 20/3=6 plays. The intermission is 20-6*3=20-18=2 tenths, or 1/5 of an hour, 12 minutes. A tenth of an hour is 6 minutes so each play lasts 18 minutes: 6 plays plus intermission gives us 6*18+12=108+12=120 minutes = 2 hours.