On one set of scales the left pan holds 4 + 9. On another the left pan holds 9 + 4. Both right pans hold 13.

Is this right? Will both scales stay level, even though the numbers have swapped places?

Look at the illustration. It shows four balanced scales. On each one, the two pans look different but stay perfectly level. Look for the pattern in each pair, what has been swapped, and what has been taken back off.

We'll balance these on the scales together, one at a time. Predict each missing block first, then say whether it came from swapping the order or from taking a number back off.

• 6 + 7 = 7 + ?
• 5 + 9 = 9 + ?
• 7 + 8 = 15, so 15 − 8 = ?
• 9 + 5 = 14, so 14 − 5 = ?

Now you try these on the scales, one at a time. Predict each missing block before we check, and say which idea you used — swapping the order or taking a number back off.