It is currently 24 Jun 2017, 19:30

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# Shortcut for multiply numbers together up to 19

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics

### Is this quicker than multiplying normally?

You may select 1 option
Author Message
TAGS:

### Hide Tags

Intern
Joined: 07 Jul 2012
Posts: 14
Shortcut for multiply numbers together up to 19 [#permalink]

### Show Tags

29 Jul 2012, 23:01
Example:

19*18=

1. Take the first number and add it with the units digit of the second number.
19+8=27

2. Multiply the result by 10
27*10=270

3. Multiply the units digit of the first number with that of the second number
9*8=72

4. Add the result of step 2 and 3 together
270+72 = 342
Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 7440
Location: Pune, India
Re: Shortcut for multiply numbers together up to 19 [#permalink]

### Show Tags

01 Aug 2012, 06:29
arnivorous wrote:
Example:

19*18=

1. Take the first number and add it with the units digit of the second number.
19+8=27

2. Multiply the result by 10
27*10=270

3. Multiply the units digit of the first number with that of the second number
9*8=72

4. Add the result of step 2 and 3 together
270+72 = 342

It would be much better to know the multiplication tables of numbers up to 19. Numbers up to 19 are used often and knowing their tables can speed up your calculations considerably
Then 18*19 is straight forward.
18*9 = 162. Write 2 and carry 16
18*1 = 18. Add 16 to get 34. You get 342.

Also, learning tables is not as hard as you might imagine.
e.g. 18*2 we know is 36. 18*3 is 18 more than 36

18*5 = 90 (12*5 = 60, 13*5 = 65, 14*5 = 70 and so on...). 18*4 is 18 less than 90. 18*6 is 18 more than 90

18*10 = 180. 18*9 is 18 less than 180.

All you really need to learn up is 18*7 and 18*8.
_________________

Karishma
Veritas Prep | GMAT Instructor
My Blog

Get started with Veritas Prep GMAT On Demand for \$199

Veritas Prep Reviews

Director
Joined: 22 Mar 2011
Posts: 612
WE: Science (Education)
Re: Shortcut for multiply numbers together up to 19 [#permalink]

### Show Tags

01 Aug 2012, 07:25
1
This post received
KUDOS
arnivorous wrote:
Example:

19*18=

1. Take the first number and add it with the units digit of the second number.
19+8=27

2. Multiply the result by 10
27*10=270

3. Multiply the units digit of the first number with that of the second number
9*8=72

4. Add the result of step 2 and 3 together
270+72 = 342

This is a translation of the algebraic multiplication of two numbers of the form 1a * 1b = (10+a)(10+b) = 100+10a+10b+ab. The latest expression we can also written it as (10+a+b)10+ab=(1a+b)10+ab.

I don't think the algorithm can be a real time saver. And its usage is quite limited (numbers below 20).
Many times, depending on the numbers, we can find specific shortcuts.
For example, 19 * 18 I would do it as (20 - 1) * 18 = 360 - 18 = 342, as it is always easy to multiply by 10 and 2.
_________________

PhD in Applied Mathematics
Love GMAT Quant questions and running.

Re: Shortcut for multiply numbers together up to 19   [#permalink] 01 Aug 2012, 07:25
Similar topics Replies Last post
Similar
Topics:
2 Time saving Divisibility Rules for numbers 1-9 6 06 Aug 2015, 06:33
7 Shortcut to find Tens digit (Last two digit) of a number 2 24 May 2017, 06:29
21 Shortcut for Odd and Even number type questions 11 18 Aug 2016, 08:41
1 SQUARE of any two digit number - shortcut 6 11 Jun 2011, 09:29
Picking up numbers 4 21 Sep 2010, 05:59
Display posts from previous: Sort by

# Shortcut for multiply numbers together up to 19

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics

 Powered by phpBB © phpBB Group and phpBB SEO Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.