How to Subtract Quickly
Quick, what’s 212 - 22? If that’s too easy for you, how about something tougher like 212 - 77? Want to learn an easy way to perform lightning fast subtraction in your head? Keep on reading to find out!
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Quick and Dirty Tips for Left-to-Right Subtraction
- Round the number you're subtracting to a convenient digit (we rounded to the nearest dollar).
- Remember how much you rounded by, and remember if you rounded up or down.
- Perform a quick mental left-to-right subtraction.
- If you rounded up, add the amount you rounded by to this result. If you rounded down, subtract it.
- Bask in the glory of your very impressive skills.
As with any new skill, to get good at mental subtraction you have to practice. So, here are a few problems for you to work through. You can find my answers and explanations below.
- 77 - 26
- 132 - 34
- 2,037 - 775
- 2,037 - 1,999
Okay, that’s all the math we have time for today. If you want to learn some more mental math techniques, check out these earlier Math Dude articles…
Be sure to check out my mental math audiobook called The Math Dude’s 5 Tips to Mastering Mental Math. And for even more math goodness, be sure to check out my book The Math Dude’s Quick and Dirty Guide to Algebra.
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Until next time, this is Jason Marshall with The Math Dude’s Quick and Dirty Tips to Make Math Easier. Thanks for reading, math fans!
Practice Problem Solutions
- 77 - 26 This problem is probably easy enough to do in your head without thinking about it. But if you want to use our left-to-right rounding method, you could round 26 up to 30 (keeping in mind that we rounded up by 4), then subtract 77 - 30 = 47, and finally adding 4 to the answer (since we rounded up) to get 77 - 26 = 51.
- 132 - 34 To solve this problem, you could round 34 down to 30 and then go through the normal process. But we could make things even quicker if we realize that 34 is simple 2 more than 32. Which means that 132 - 34 is the same as 132 - 32 - 2, which of course is equal to 98.
- 2,037 - 775 We can round 775 up to 800 (remember that we've rounded up by 25), subtract 800 from 2,037 (which, with a bit of practice, you'll be able to quickly figure out is 1,237), and then add 25 to this result to find the final answer: 2,037 - 775 = 1,237 + 25 = 1,262.
- 2,037 - 1,999 The trick here is to realize that 1,999 is just 1 less than 2,000. Which immediately tells us that 2,037 - 1,999 must be equal to 38.
The point with all of these is that a bit of quick thinking before acting along with a little simple rounding allows you to solve the problems from left-to-right. Do you see how much easier that makes things? If not, try doing the problem 2,037 - 1,999 using the old-fashioned right-to-left method—and be sure to do all of the carrying. You should find that it's a lot more work…and it's work that you simply don't need to do!
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