Look at these two cube towers. One is 2 red cubes joined to 9 blue cubes. The other is the same tower flipped end over end.

Did the tower get any taller when we turned it around?

Look at the two number lines. The top one starts on 2 and makes 9 little jumps. The bottom one starts on 9 and makes 2 little jumps. Where does each one land?

They land on the same number. 2 + 9 and 9 + 2 are a turnaround: the same two numbers, swapped, like the cube tower turned round.

Now 4 + 13 and 13 + 4. Which one has fewer jumps? Do they land on the same number?

The total stays the same, so we stand on the bigger number and make fewer jumps.

Turn it round! For each sum, say the bigger number and put it in your head. Then count on the smaller number on your fingers. The last number you say is where you land.

We will do 3 + 9, then 2 + 11, then 4 + 10, then 1 + 15.