Try and check

Primary 3 More Word Problems · Lesson video · 5 min 55 s

Try, check the total, then improve the try. Keep the number of bags the same every time.

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Questions in this lesson

  1. 0:21 Today's question: Mei has 10 bags of shells. Each small bag has 2 shells. Each large bag has 4 shells.
  2. 2:28 Example 1: Siti has 9 bags of shells. Each small bag has 2 shells. Each large bag has 5 shells.
  3. 4:17 Example 2: Ravi has 12 bags of shells. Each small bag has 3 shells. Each large bag has 5 shells.
Transcript

In Primary 2, you learned that 5 bags of 2 shells make 5 times 2, which is 10 shells. Today, we will try, check and improve, using a table. Mei has 10 bags of shells. Each small bag has 2 shells. Each large bag has 4 shells. There are 26 shells altogether. How many large bags are there? Altogether there are 10 bags. We do not know how many of each yet, so we start with a try. Let us try, check and improve, using a table. Let us try 5 large bags and 5 small bags. 5 times 2 is 10, and 5 times 4 is 20. 10 plus 20 is 30. We need 26, so 30 is too many. A large bag has more shells. To get fewer shells, we use fewer large bags. We still need 10 bags, so 1 large bag becomes a small bag. 6 small bags and 4 large bags. 6 times 2 is 12, and 4 times 4 is 16. 12 plus 16 is 28. Still too many, but closer. 1 change gave us 2 fewer shells. We need 2 fewer again, so we change 1 more bag. 7 small bags and 3 large bags. 7 times 2 is 14, and 3 times 4 is 12. 14 plus 12 is 26. That is the total we need. So there are 3 large bags.

Siti has 9 bags of shells. Each small bag has 2 shells. Each large bag has 5 shells. There are 33 shells altogether. How many large bags are there? Try it first. Pause if you like. Use this table. 9 bags cannot be split in half. Try 6 small and 3 large. 6 times 2 is 12, and 3 times 5 is 15. 12 plus 15 is 27, too few. 27 is too few. A large bag has more shells, so try 1 more large bag and 1 fewer small bag. 5 small and 4 large. 5 times 2 is 10, and 4 times 5 is 20. 10 plus 20 is 30, still too few. Still too few, so try 1 more large bag and 1 fewer small bag again. 4 small and 5 large. 4 times 2 is 8, and 5 times 5 is 25. 8 plus 25 is 33. That is the total we need. So there are 5 large bags.

Ravi has 12 bags of shells. Each small bag has 3 shells. Each large bag has 5 shells. There are 46 shells altogether. How many small bags are there? Try it first. Pause if you like. Use this table. 6 small and 6 large. 6 times 3 is 18, and 6 times 5 is 30. 18 plus 30 is 48, too many. 48 is 2 too many. Changing 1 large bag into a small bag gives 2 fewer shells. 7 small and 5 large. 7 times 3 is 21, and 5 times 5 is 25. 21 plus 25 is 46. That is the total we need. So there are 7 small bags. Try, check the total, then improve the try. Keep the number of bags the same every time.