Round off to the nearest ten:
To round off a number to the nearest ten, we look at the digit in the ones place. If the digit is 0, 1, 2, 3, or 4, we round down to the nearest ten. If the digit is 5, 6, 7, 8, or 9, we round up to the nearest ten.

- Rounding off 642 to the nearest ten:
The digit in the ones place of 642 is 2, which is less than 5. Therefore, when rounding off 642 to the nearest ten, we round down. So, 642 rounded off to the nearest ten is 640.

- Rounding off 278 to the nearest ten:
The digit in the ones place of 278 is 8, which is greater than 5. Therefore, when rounding off 278 to the nearest ten, we round up. So, 278 rounded off to the nearest ten is 280.

Find the difference:
To find the difference between two numbers, we subtract the smaller number from the larger number.

- Difference between 642 and 278:
642 - 278 = 364

Round off the difference to the nearest hundred:
To round off a number to the nearest hundred, we look at the digit in the tens place. If the digit in the tens place is less than 5, we round down to the nearest hundred. If the digit in the tens place is 5 or greater, we round up to the nearest hundred.

- Rounding off 364 to the nearest hundred:
The digit in the tens place of 364 is 6, which is greater than 5. Therefore, when rounding off 364 to the nearest hundred, we round up. So, 364 rounded off to the nearest hundred is 400.

Summary:
- Rounding off 642 to the nearest ten: 640
- Rounding off 278 to the nearest ten: 280
- Difference between 642 and 278: 364
- Rounding off 364 to the nearest hundred: 400

Therefore, the difference between rounding off 642 and 278 to the nearest ten is 40, and when rounded off to the nearest hundred, it is 400.