Idea 13of 23Builds onAdd in any order
Group to make a friendly number
How can you add three big numbers in your head?
Today you'll discover
- how to spot a pair that makes thousands
- why grouping keeps the total
- how to finish the sum
Think first2 750 + 4 600 + 1 250
In 2 750 + 4 600 + 1 250, which two would you add first?
Decide first, then press Next. Look for endings that make 1 000.
A bar split into pieces of 2 750, 4 600 and 1 250, with a question mark for the total. A second bar under it moves the 1 250 next to the 2 750, with a bracket marked first: 4 000. Then 2 750 + 1 250 = 4 000 and 4 000 + 4 600 = 8 600. The last step shows 1 450, 3 800 and 550.
See the whole ideathe big idea and the key word, in one place
The big idea
Add three numbers in any grouping: pick a pair that makes a friendly number first.
For exampleThree bake-sale tins hold $1 450, $3 800 and $550. Add $1 450 and $550 first to get $2 000, then $5 800 in all.
Key words
- associative property
- when you add three numbers, you can group them any way and the total stays the same
- friendly number
- a number that is easy to work with, like 4 000 or 10 000
Ready to use it?
- Choose which two numbers to add first to make a friendly number.