Addition bridging through 10 (within 20)
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Lan has sweets. Her mum gives her more. How many sweets does Lan have altogether?
You could count on one at a time: . But counting like that is slow and easy to get wrong. This lesson shows you a much quicker trick.
Let's explore
Lan has a sweet box with exactly spaces. She puts her sweets in, so spaces are still empty.
- Lan takes of the sweets from her mum to fill the box. Now the box has .
- of mum's sweets are left outside the box.
- in the box and outside make .
So . You see: just make 10 first, and the answer is easy.
The trick works because no sweet is lost: you only move of them into the box to make , and the rest are still counted.
Examples
We start with a sum that has a in it, because needs only more to make .
With a , you need more to make .
When the first number is the smaller one, you can swap the two numbers to make it easier.
Now use addition to answer a question from everyday life.
Key terms
Summary
- To add across , make 10: split the second number into the part that makes and the part that is left.
- Example: .
- Swapping the addends does not change the sum; add on to the bigger number to make it easier.
- Questions with "altogether" or "both" usually need addition; remember the unit in the answer.
Practice
Each exercise comes with what you need and how to start. Try it yourself first, then open the solution.
Work out .
What you need: make : needs more.
Steps:
- Split into and .
- Work out , then add .
Show solution
.
Answer. .
Work out .
What you need: needs more to make .
Steps:
- Split into and .
- Work out , then add what is left.
Show solution
.
Answer. .
Work out .
What you need: needs more to make .
Steps:
- Split into and .
- Work out , then add .
Show solution
.
Answer. .
Work out .
What you need: swapping the two addends does not change the sum.
Steps:
- Write as .
- needs more: split into and .
Show solution
.
Answer. .
Write , or in the box: .
What you need: work out the sum first, then compare the two numbers.
Steps:
- Work out with the make-10 trick.
- Compare the result with .
Show solution
. Since , we have .
Answer. The sign .
In a garden there are hens and cockerels. How many chickens are there altogether?
What you need: "altogether" means putting together: use addition.
Steps:
- What do we know? hens and cockerels. What is asked? How many chickens altogether.
- Work out and write the answer with its unit.
Show solution
The number of chickens altogether is:
Answer. chickens.
Remember the last lesson: what number comes just after ? Then work out (the number just after ) .
What you need: the number just after a number is more than it.
Steps:
- Find the number just after .
- Add to it (count on, or add the ones).
Show solution
The number just after is . Then (because , and and make ).
Answer. .
Find the missing number: .
What you need: needs more to make ; then how many more to reach ?
Steps:
- From to you need .
- From to you need .
- Add the two parts: .
Show solution
From up to is , and from up to is , so the missing number is .
Check. . Correct.
Answer. .
Mini test
Marked like the Vietnamese graduation exam: Part I 0.25 points per question; Part II up to 1 point per question (1, 2, 3, 4 correct statements give 0.1, 0.25, 0.5, 1 point); Part III 0.5 points per question.
Part I. Multiple choice
. Option D is the mistake of forgetting the that is left.
needs more to make , so split . Option D is wrong because , not .
. The other sums are , and .
"Altogether" means putting together: . Option A is , a subtraction by mistake.
Part II. True or false
| a) It can be written as without changing the answer. | ||
| b) . | ||
| c) . | ||
| d) is greater than . |
a) True: swapping the addends does not change the sum. b) True: . c) False: . d) True: .
Part III. Short answer
.
(birds).
Show answers and explanations
- Part I, question 1: B. . Option D is the mistake of forgetting the that is left.
- Part I, question 2: C. needs more to make , so split . Option D is wrong because , not .
- Part I, question 3: A. . The other sums are , and .
- Part I, question 4: C. "Altogether" means putting together: . Option A is , a subtraction by mistake.
- Part II, question 1: a) True; b) True; c) False; d) True. a) True: swapping the addends does not change the sum. b) True: . c) False: . d) True: .
- Part III, question 1: 18. .
- Part III, question 2: 11. (birds).