You have a frying pan which will take only 2 slices of bread at a time, and you wish to fry 3 slices, each on both sides. Since each slice takes 20 seconds for each side, you can certainly fry them all in 80 seconds, by doing 2 pieces together and then the third. Can you do them more efficiently?

The key insight is that you don't need to finish both slices completely before removing one from the pan. Optimal strategy: first 20 seconds cook sides A of slices 1 and 2; next 20 seconds flip slice 1 to side B and swap slice 2 out for slice 3 (side A); final 20 seconds cook sides B of slices 2 and 3. Total time: 60 seconds, significantly better than 80.