Getting the answer is half the job. Knowing why it works is the other half.
Step 1: Let's Learn
Read it, or press Listen and follow the words.
The reason is always the same
You may only add things of the same size. Tens with tens, ones with ones.
And bundling never changes the amount
Ten ones and one ten are the same amount, so swapping them is allowed.
This catches mistakes
If someone adds a ten to a one, the units do not match, and the answer will be wrong.
Why explaining is part of the maths
Getting the right answer shows you can follow a method. Explaining why the method is allowed shows you understand it. That understanding is what lets you spot your own mistakes and what lets you handle a problem you have not seen before.
Words that make an explanation clear
Useful phrases: "I added the tens first because...", "I bundled ten ones into one ten because...", "I know this is right because...". Naming the units — ones, tens, hundreds — rather than saying "the 4" makes an explanation far easier for someone else to follow.
A drawing is an explanation too
A picture of blocks with an arrow showing ten ones becoming one ten explains the carry better than a paragraph. You are allowed to explain with a diagram, as long as someone else can read your reasoning from it.
Step 2: Try It Yourself
Tap and try it out.
First number
and Second number
Altogether
Tens
7
Ones
13
47 + 36 = 83
There are 13 loose ones. Ten of them can become one more ten.
Step 3: Watch an Example
One step at a time.
Watch Leo Explain
Leo has just worked out 47 + 36 = 83, and is asked why.
- Step 1
He says the tens joined the tens: 4 and 3 make 7 tens.
Step 4: Your Turn
Practice makes it stick.
Spot the Mistake
Problem 1 of 2
Someone works out 34 + 25 and gets 95. What went wrong?
Why Bundle?
Problem 2 of 2
Why is it allowed to swap ten ones for one ten?
Why Does It Work?
1 of 3
Why do you add the tens to the tens?
2 of 3
48 + 27 = ?
3 of 3
In 48 + 27, how many loose ones are there before bundling?
Step 5: Quick Check
Show what you know.
Question 1 of 2
Someone says the bundled ten is "a one". Are they right?
Question 2 of 2
56 + 38 = ?
What You Learned
- You may only add units of the same size.
- Bundling is allowed because the amount does not change.
- Explaining why is how you catch your own mistakes.