ôô By
Jason Marshall, PhD
Episode #8

## How to Add Quickly, Part 1 ## Tip #1: Find Pairs of Numbers that Add to 10

Here is perhaps the single most useful quick and dirty tip to help you calculate quickly: When adding a list of numbers, look for pairs of numbers that add to ten. Here’s what I mean:

1 + 9,
2 + 8,
3 + 7,
4 + 6,
5 + 5.

All of those pairs add to ten.

So, when you’re adding groups of numbers, look for these special pairs. If you’ve got one or more pairs, it’s easy to add them up—as easy as 10 + 10 = 20.

## How to Do It

Here’s an example: Let’s say you need to add the numbers 1, 3, 5, 7, and 9. You could run through the numbers in order, adding them up one-by-one; something like:

1 + 3 = 4, then
4 + 5 = 9, then
9 + 7 = 16, and finally
16 + 9 = 25.

But that’s a lot to keep track of. Remember the commutative property of addition? You don't have to add these numbers in the order they're given to you. Instead, if you start by pairing up numbers that add to ten—in this case 3 and 7, and 9 and 1—that leaves you with an easy problem. You just have to add the two 10s to the remaining number 5 that didn’t pair with anything:

10 + 10 + 5 = 25

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