# Avoiding the carry **URL:** \ **Category:** Mental Calculation, Mathematics, and Puzzles\ **Created:** [August 27, 2014, 12:02pm UTC](https://forum.artofmemory.com/t/avoiding-the-carry/29784 "2014-08-27T12:02:46Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Kinma](https://forum.artofmemory.com/letter_avatar_proxy/v4/letter/k/a88e4f/32.png) [@Kinma](https://forum.artofmemory.com/u/Kinma)\ **Post date:** [August 27, 2014, 12:02pm UTC](https://forum.artofmemory.com/t/avoiding-the-carry/29784/1 "2014-08-27T12:02:46Z")
One of the difficult things in mental calculation is to keep track of a carry when you are juggling numbers. In a lot of cases we can avoid the carry. An example will make this clear, I hope. Let’s say we need to calculate: 79 89 \_\_X Doing this calculation the traditional way leads to a lot of carrying. However: 79 is pronounced ‘seventy nine’, meaning 70 + 9. Instead of this, try to see 79 as ‘80 minus one’, which is the same number of course. 89 then becomes ‘90-1’. Let’s change the calculation to reflect this. We have ‘80 - 1’ times ‘90 - 1’. I will add extra spacing, but try to see the 8 as 80 and the 9 as 90: 8 - 1 9 - 1 \_\_\_\_\_X Now, if we do left to right cross multiplication we get: 1: 80 X 90 = 7200 2: -80 - 90= - 170 (or -1 X 80 -1 X 90) Subtract 170 from 7200 to get: 7030 3: -1 X - 1 = 1 Add to 7030 = 7031. Done. (Technically, in subtracting 170 from 7200 there is a small carry, but most people will hardly notice this and immediately see that 200 - 170= 30.) I hope you see that this makes the calculation a lot easier. For practice, start by trying to see ‘38’ as ‘40-2’. And ‘67’ as ‘70-3’. Then mentally multiply them. ---
**Author:** ![Kinma](https://forum.artofmemory.com/letter_avatar_proxy/v4/letter/k/a88e4f/32.png) [@Kinma](https://forum.artofmemory.com/u/Kinma)\ **Post date:** [August 28, 2014, 1:42pm UTC](https://forum.artofmemory.com/t/avoiding-the-carry/29784/2 "2014-08-28T13:42:05Z")
This technique is especially handy when we multiply number close to a round number. Example: 38 42 \_\_X Try to do the cross multiplication and notice the amount of carrying. Now try to see it as: 4 -2 4 2 \_\_\_\_X The second step of the cross multiplication cancel each other out (-2X4 +2X4). We are left with 1600 - 4, which is easy to do mentally. ---
**Author:** ![anon\_bot](https://forum.artofmemory.com/user_avatar/forum.artofmemory.com/anon_bot/32/30320_2.png) [@anon\_bot](https://forum.artofmemory.com/u/anon_bot)\ **Post date:** [July 1, 2015, 2:49am UTC](https://forum.artofmemory.com/t/avoiding-the-carry/29784/3 "2015-07-01T02:49:43Z")
[this post was removed by request] ---
**Author:** ![Tiger](https://forum.artofmemory.com/letter_avatar_proxy/v4/letter/t/a8b319/32.png) [@Tiger](https://forum.artofmemory.com/u/Tiger)\ **Post date:** [July 7, 2017, 6:29am UTC](https://forum.artofmemory.com/t/avoiding-the-carry/29784/4 "2017-07-07T06:29:48Z")
This is terrific! What a difference it makes with 38 x 42. I like the cross multiplication method, my only problem with it is remembering the original numbers. I am trying to do all calculations mentally without trying to write anything down. ---
**Author:** ![greymatters](https://forum.artofmemory.com/user_avatar/forum.artofmemory.com/greymatters/32/2684_2.png) [@greymatters](https://forum.artofmemory.com/u/greymatters)\ **Post date:** [July 8, 2017, 2:37pm UTC](https://forum.artofmemory.com/t/avoiding-the-carry/29784/5 "2017-07-08T14:37:55Z")
> [@](#): > > Technically, in subtracting 170 from 7200 there is a small carry, but most people will hardly notice this and immediately see that 200 - 170= 30. Or, just work from left to right: 7200 - 170 = 7200 - 100 - 70 ---
**Author:** ![Pinkesh](https://forum.artofmemory.com/letter_avatar_proxy/v4/letter/p/b782af/32.png) [@Pinkesh](https://forum.artofmemory.com/u/Pinkesh)\ **Post date:** [October 2, 2019, 9:14am UTC](https://forum.artofmemory.com/t/avoiding-the-carry/29784/6 "2019-10-02T09:14:44Z")
It’s a wrong 22 ×38 =836 3-8 3 8 =-24+11 =3500-4=3496 ---
**Author:** ![bjoern.gumboldt](https://forum.artofmemory.com/user_avatar/forum.artofmemory.com/bjoern.gumboldt/32/16747_2.png) [@bjoern.gumboldt](https://forum.artofmemory.com/u/bjoern.gumboldt)\ **Post date:** [October 2, 2019, 11:24am UTC](https://forum.artofmemory.com/t/avoiding-the-carry/29784/7 "2019-10-02T11:24:54Z")
> [@Pinkesh](#): > > 3-8 > 3 8 > =-24+11 > =3500-4=3496 How do you get to 3500? Your setup is right: 1. 3x3=9 or 30x30= **900** if you prefer 2. cross multiplication **cancels out** (-24+24) 3. -8x8= **-64** Gives you **900-64=836** ---
**Author:** ![Pinkesh](https://forum.artofmemory.com/letter_avatar_proxy/v4/letter/p/b782af/32.png) [@Pinkesh](https://forum.artofmemory.com/u/Pinkesh)\ **Post date:** [October 2, 2019, 6:33pm UTC](https://forum.artofmemory.com/t/avoiding-the-carry/29784/8 "2019-10-02T18:33:24Z")
Thank you ---
**Author:** ![\_Shadow01](https://forum.artofmemory.com/user_avatar/forum.artofmemory.com/_shadow01/32/11034_2.png) [@\_Shadow01](https://forum.artofmemory.com/u/_Shadow01)\ **Post date:** [November 25, 2021, 3:38am UTC](https://forum.artofmemory.com/t/avoiding-the-carry/29784/9 "2021-11-25T03:38:30Z")
tbh i just answering this randomly and its probs wrong because I know like no mental math methods but how @kinma describes this is that the question becomes 40| -2 x40|+2 so 4x4=16 x 100 1600 then you -2x38 or 76 so 1524 then you add 42x2 or 84 which becomes 1604 then you add -2 x 2 or -4 which becomes 1600 but thats wrong because using the one method I know I can work out that it is 1596 because (30 x 42=1260 then add 8 x 42 and you get ( 40 x 8 = 320 + 16=336) 1596. this is something I find hard so im trying to get used to mental math but noone seems to be truely helpful. anyway I believe kinma explained it but I got confused lol. it seems to add more to the question ---
**Author:** ![Rajadodve786](https://forum.artofmemory.com/user_avatar/forum.artofmemory.com/rajadodve786/32/32911_2.png) [@Rajadodve786](https://forum.artofmemory.com/u/Rajadodve786)\ **Post date:** [November 25, 2021, 5:31am UTC](https://forum.artofmemory.com/t/avoiding-the-carry/29784/10 "2021-11-25T05:31:38Z")
@_Shadow01 38 × 42 there is difference of 4 numbers between them. Square the middle and subtract 4 (2²) 40² - 4 =\> 1600 - 4 =\> 1596. ---
**Author:** ![\_Shadow01](https://forum.artofmemory.com/user_avatar/forum.artofmemory.com/_shadow01/32/11034_2.png) [@\_Shadow01](https://forum.artofmemory.com/u/_Shadow01)\ **Post date:** [November 25, 2021, 8:44am UTC](https://forum.artofmemory.com/t/avoiding-the-carry/29784/11 "2021-11-25T08:44:01Z")
thanks that helps. I actually kind of understand so yours helped more for me than kinmas. thank you