What time must a roast be placed in the oven to be ready by 5:30 if the roast takes 2 hours and 40 minutes to cook?

2 hours 40 mins = 2 hours + 30 mins + 10 mins

So we will break it into 3 steps (1) 2 hours (2) 30 mins (3) 10 mins

Step (1):

5:30 → Count back 2 hours = 3.30

Step (2):

3.30 → Count back 30 mins = 3.00

Step (3):

3.00 → Count back 10 mins = 2:50

Answer: The roast needs to be in the oven at 2:50