Question

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?

Answer 1

5:30 - 2:40 = (5 -2):(30 -40) = 3:(-10) = 2:50

The roast must go into the oven at 2:50.

Answer 2

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