Learn two quick and dirty tips to help you kick your calculator dependency and make faster calculations in your head.
Tip #2: Find Groups of Numbers that Add to 10
The second tip is really an addendum to the first. Say you need to add a series of numbers like 47, 14, 21, and 32. Let’s look for pairs of numbers in the ones place that add up to 10. That's the 7 from 47, the 4 from 14, the 1 from 21, and the 2 from 32. Hmm, no two numbers here add up to 10. Are we out of luck?
No, look at the numbers again: 7, 4, 1, and 2. Three of those numbers do add up to ten:
7 + 1 + 2 = 10
That brings us to the second quick and dirty tip: Don’t restrict yourself to grouping pairs of numbers—sometimes looking at groups of three (or even more) can help.
How to Do It
Let’s look at how to solve the example problem from before in a little more detail: 47 + 14 + 21 + 32. First, notice that each term in the problem can be written as:
47 = 40 + 7,
14 = 10 + 4,
21 = 20 + 1, and
32 = 30 + 2.
Then, let’s use this to rewrite the original problem:
Now, as we discussed during the episode on the commutative property of addition, you can add these numbers in any order you like. So, let’s gather the numbers that add up to 10 and rearrange the terms as:
The three numbers in the first term add to 10 and the four numbers in the second term add to 100, so the problem becomes:
Believe it or not, eventually—after a bit of practice—you’ll be able to do all this in your head.