Cái cân, so sánh bắc cầu, đổi vật tương đương

1. What a balance tells you

A balance has one pan at each end. The lower pan holds the heavier side. The higher pan holds the lighter side. If the pans are level, the two sides weigh the same. Always look at the pans before you answer.

2. Linking comparisons

A question often gives two separate comparisons. For example a mango is heavier than an orange. The orange is heavier than an apple. So the mango must be heavier than the apple. This is called linking, or transitivity.

3. Put them in one line

The safest way is to line the objects up. Write them from the lightest to the heaviest. In the example the line is apple, orange, mango. The line answers every question for you. The object at the end of the line is the heaviest.

4. Equal swaps

A question may say 1 melon balances 3 oranges. It also says 1 orange balances 2 grapes. So each of the 3 oranges swaps for 2 grapes. Add 2 plus 2 plus 2 to get 6. One melon swaps for 6 grapes.

5. Swapping in several steps

Some questions link up to three balances. Swap step by step, from the big item down to the small one. Write down the result of each step. Do not skip a step: it is easy to slip. One wrong step spoils the whole answer.

6. Check your answer

Put your answer back into each balance. If every balance still works, your answer is right. Read the question again to see which item it asks about. Many children work it out but name the wrong item.