Three-digit adding and taking away · Usually met around P2
Adding in a handy order
Add the two that make a hundred first.
How to do it
Method
Add the two that make a hundred first
1 + 65 + 399 = 65 + 400 = 465
Another way
Left to right
1 + 65 = 66, 66 + 399 = 465
In full
Adding works in any order: 65 + 1 = 1 + 65, the same answer. With three numbers you may add any two first. So look for two numbers that make a whole ten or hundred and add those first: 1 + 65 + 399, first 1 + 399 = 400, then 65 + 400 = 465, easier than going left to right. Pairs that make a hundred: the ones make 10 and the tens make 9, like 37 and 63. Or add left to right as usual, to check.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
17 + 578 + 83 = □
Show the answer and working
Answer: 678
Add 17 and 83 first to make 100; 578 + 100 = 678.
Example 2
266 + 49 + 34 = □
Show the answer and working
Answer: 349
Add 266 and 34 first to make 300; 300 + 49 = 349.
Example 3
398 + 39 + 244 = 39 + 244 + 408
True or false?
Show the answer and working
Answer: False
Adding works in any order, but only if the right side has exactly the same three numbers as the left.
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