GoalAdd lengths in centimetres, find a length that is more than another, and check a product for mistakes.
New words
longer than · greater in length than anothertotal length · all the lengths put togethercheck · to test an answer in another way
Explanation
When one rope is longer than another by some centimetres, we add. If rope A is 47 cm and rope B is 15 cm longer, then rope B is 47 + 15 = 62 cm. Add the ones first: 7 + 5 = 12, write 2 and carry 1. Then add the tens: 4 + 1 + 1 = 6. The total length of both ropes is 47 + 62 = 109 cm, which is 1 m 9 cm. We should also check products. If someone writes 4 × 5 = 9, they added instead of multiplying. Draw 4 groups of 5 and count: 5 + 5 + 5 + 5 = 20. So 4 × 5 = 20. Repeated addition, an array or a fact family can show us when a product is wrong.
Place
Work
Write
Ones
7 + 5 = 12
2, carry 1
Tens
4 + 1 + 1 = 6
6
Answer
62 cm
Worked examples
Rope A is 47 cm. Rope B is 15 cm longer. Rope B is 47 + 15 = 62 cm. Together the ropes are 47 + 62 = 109 cm.
Mostafa writes 4 × 5 = 9. He added 4 + 5. Count the 4 equal groups of 5 to check: 5 + 5 + 5 + 5 = 20. The right answer is 4 × 5 = 20.
Way to check
Work
Result
Nearby fact
6 × 6 + 6
42
Repeated addition
7 + 7 + 7 + 7 + 7 + 7
42
Mina says 6 × 7 = 48. Check with a nearby fact: 6 × 6 = 36, and one more group of 6 makes 36 + 6 = 42. So 6 × 7 = 42, and Mina made a mistake.
Class activity
Rope lengths. Cut two pieces of string, for example 38 cm and another that you measure to be 25 cm longer. Add to find the second length and check with a ruler. Then find the total length of both strings. Finally write three multiplications and make one wrong on purpose for a partner to find and fix.
Practice
1
A blue ribbon is 38 cm long. A red ribbon is 25 cm longer. How long is the red ribbon?
38 + 25 = 63 cm.
2
Two planks are 56 cm and 36 cm long. What is the total length when they lie end to end?
56 + 36 = 92 cm.
3
Karim wrote 3 × 8 = 11. What mistake did he make? What is the right answer?
He added 3 + 8. The product is 3 × 8 = 24.
4
Salma wrote 7 × 4 = 24. Check with repeated addition. Is she right?
7 × 4 means 4 groups of 7: 7 + 7 + 7 + 7 = 28, so 7 × 4 = 28. Salma is not right.
5
A string is 65 cm. Another string is 28 cm longer. What is the length of both together? Is it more than 1 m?
The second string is 65 + 28 = 93 cm. Together: 65 + 93 = 158 cm. Yes, 158 cm is more than 1 m (100 cm).
