In a before-and-after problem, items move between two groups but the total stays the same — nothing is added or thrown away. The trick is to compare the before and after using that common total. For example, when Aiden gives some stamps to Bina, together they still have the same number as before. Drawing a before bar and an after bar of equal length helps you see the change.
Worked example
Problem Mei and Jun have 120 stickers altogether. Mei : Jun = 7 : 3, then after Mei gives some to Jun it becomes 3 : 2. How many did Mei give?
The total (120) does not change, so use it for both before and after.
Before: 7 + 3 = 10 units, so 1 unit = 120 ÷ 10 = 12. Mei had 7 × 12 = 84.
After: 3 + 2 = 5 units, so 1 unit = 120 ÷ 5 = 24. Mei has 3 × 24 = 72.
Mei gave 84 − 72 = 12 stickers.
Watch out One unit is a different size before and after — the total is fixed, but the number of parts changes.