The big ideaAdd back what you took away. If you land on the start, it is right.
How it works
Adding and taking away undo each other, like putting four blocks on and taking four off.
After a take away, add back what you took away. If you land on the start, your answer is right.
It works the other way too: to check an add, take a part away, and you land on the other part.
A check takes only a moment, and it catches a slip before it grows.
Check by adding back.
Example 1: A check that works
There were 71 beads and 24 were sold: 71 - 24 = 47.
Add the sold beads back: 47 + 24 = 71.
You landed on 71, so 47 is right.
Adding back lands on 71.
Example 2: Catch a slip
Ben says 83 - 49 = 46. Add back: 46 + 49 = 95.
95 is not 83, so Ben has a slip: 3 ones cannot take away 9, so he had to borrow.
The right answer is 34, and 34 + 49 = 83.
The right answer, 34.
Example 3: Check an add
38 + 27 = 65.
Take a part away to check: 65 - 27 = 38.
You land on the other part, 38, so the add is right.
The total, 65.
Example 4: A family of three
The numbers 71, 24 and 47 belong together.
24 + 47 = 71
47 + 24 = 71
71 - 24 = 47
71 - 47 = 24
Words to know
add back
Add on what you took away, to see if you land on the start.
check
A second way to see if an answer is right.
undo
Adding and taking away undo each other.
slip
A small mistake, like a missed borrow.
Watch out
Add back the part you took away, not the start.
If you do not land on the start, hunt for the slip. A borrow is the usual one.
Check the sums that matter. It only takes a moment.
Try it
Check 91 - 36 = 55. What is 55 + 36?
Ben says 75 - 38 = 43. What is 43 + 38?
What is 75 - 38?
Which sum checks 72 - 38 = 34, 34 + 38 or 34 + 72?
Answers: 1. 91 · 2. 81 · 3. 37 · 4. 34 + 38
Try at homeAsk a grown-up to give you a take away sum, like the number of pages in a book you are reading. Work it out, then add the part you took away back on. Did you land on the start? If not, hunt for the slip together.
For grown-upsGive your child a take away sum from real life, such as pages left in a book. Work it out, then add the part back and check you land on the start.
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