When you add two numbers, the order doesn't matter. Whether you do a + b or b + a, you land on the very same total:

This is the commutative property of addition. It means you can always swap the two numbers around and the answer stays put.

Watch both orders at once. The top number line starts at the first number and hops on by the second; the bottom one starts at the second number and hops on by the first. Each time you replay it picks new numbers — but both markers always finish on the same total.

That's why it pays to start with the bigger number: to work out 2 + 9, begin at 9 and count on just two — much quicker than starting at 2 and counting on nine. The answer is the same either way.

Khan Academy explains the commutative law of addition here: