Adding pairs to 100 and 1000 method

I decided to upload the method we used in class for adding pairs to 100 and 1000.

I have this sum: 730 + _ = 1000. 

Step 1:
Find how many more to arrive to 100. So, how many numbers do you need so that from 73 you arrive to 100?

73 + _ = 100

Step 2:
Look at the unit digit number. It is 3. How many more to 10? The answer is 7. So, if I need 7 to make 10, than I need 7 to make the next ten of 73 which is 80. So:

73 + 7 = 80.

Step 3:
Now, I need to arrive to 100. How many more to arrive to 100? 20 more. So,

80 + 20 = 100.

Step 4:
How many numbers did I add altogether? 7 + 20 = 27. 

Step 4:
Now we know that you need 27 more with 73 to make 100. So,
73 + 27 = 100.

If you arrived there, now you can write the matching pair to make 1000. In class I told you that each number in a pair totalling 1000 is ten times the matching pair totalling 100. So therefore, if

73 + 27 = 100, than
730 + 270 = 1000.