Two ways to decompose this number is:
749 = 700+40+9; and
749 = 100+649.
Explanation:
One method to decompose a number is to break it up by place value, into hundreds, tens and ones.
In this number, 7 is in the hundreds place; this makes 7(100) = 700.
4 is in the tens place; 4(10) = 40.
9 is in the ones place; 9(1) = 9.
This makes 749 = 700+40+9.
Another method to decompose a number is to break it into pieces that can be added together.
For this number, I chose to start with 100. This means the second addend would be 749-100 = 649; this gives us
749 = 100+649.