20. A roast requires 3 hours and 40 minutes to prepare. When must it be put into the oven in order to be ready to be served at 6:30 P. M.?

6:30
1 hour=60 minutes
60x6=360+30=390
60x3=180+40=220
390-220=170

170 divided by 60=2.83

2x60=120

170-120=50

2:50

Check:

2:50+60=3:50+60=4:50+60=5:50+40=6:30
60+60+60+40=220

To roast a food item in the oven that takes 3 hours and 40 minutes to roast and have it done and served by 6:30 p.m, you would have to start at 2:50 p.m.

2:50 p.m.

His goal is  achievable, realistic, measurable, and time-based. : )