|Total GMAT Math
Jeff's complete Quant guide, on sale now!
|Total GMAT Verbal
Everything you need to ace GMAT Verbal!
1,800 Practice Math Questions
Buy Jeff's books at Amazon.com
GMAT Official Guide, with IR
OG Math | OG Verbal
OG12 & Quant Rev solutions!
GMAT Question of the Day
Beginner's Guide to the GMAT
GMAT Hacks Affiliate Program
- General Study Tips
- Goals and Planning
- CAT Strategy
- The Mental Game
- GMAT Math Strategy
- GMAT Math Topics
- Mental Math
- Data Sufficiency
- Critical Reasoning
- Reading Comprehension
- Sentence Correction
- Analytical Writing Assessment
- Integrated Reasoning
- IR Explained
- Business School Admissions
- GMAT Prep Resources
- Practice Questions
- Total GMAT Math
- Total GMAT Verbal
- GMAT 111
Mental Math: Multiply By Nine (and other numbers too!)
|You should follow me on Twitter. While you're at it, take a moment to subscribe to GMAT Hacks via RSS or Email.|
If you've read many of the articles on this site, you know how passionately I feel about avoiding traditional calculation methods. Here's a way to multiply large numbers (or any numbers, really) by 9, without setting up multi-digit multiplication problems. You'll save time, too.
Since 9 is the same as (10-1), you can substitute the latter term anywhere you see a 9:
23(9) = 23(10 - 1)
Presumably, you can multiply 23 times 10 quite rapidly, and 23 times 1 even faster:
23(10 - 1) = 230 - 23 = 207
This is very similar to the method I introduced a while back for percents. And like the method for percents, you can use it for numbers other than 9, too. The idea is: if you're multiplying by a number that is the sum or difference between two easier numbers, you can turn that one multiplication problem into 2. For instance:
19(8) = 19(10 - 2) = 190 - 38 = 152
27(11) = 27(10 + 1) = 270 + 27 = 297
25(15) = 25(10 + 5) = 250 + 125 = 375
20(99) = 20(100 - 1) = 2000 - 20 = 1980
Try it on some other multiplication problems yourself! With a bit of practice, you'll get through calculations like these much more quickly than before.
About the author: Jeff Sackmann has written many GMAT preparation books, including the popular Total GMAT Math, Total GMAT Verbal, and GMAT 111. He has also created explanations for problems in The Official Guide, as well as 1,800 practice GMAT math questions.
|Total GMAT Math
The comprehensive guide to the GMAT Quant section. It's "far and away the best study material
available," including over 300 realistic practice questions and more than 500 exercises!