Mental Arithmatic, help anyone?

[Peachface]

I found this partial mental arithmatic maths formula, for multiplying numbers in your head, on an old thread. I can only get it to work for numbers below 20. If someone can decipher it into a practical form, and get it to work for larger numbers, it would be a huge advantage when the calculator is out of arms reach.

This the original fragment,

Say 14 x 13...mutiply the 4X3 if theres any remainders then add that on to the total of 4+3 and add the first digits. So.. 4X3 = 12..3+4+1(carry the ten remainder)=8 and 1+1 =1 so equals 182

OR

14*13...4*3=12
............4+3+1=8 (1 is the remainder from the first line)
............1*1+0=1 (0 because 8 has no remainder)
=182

It sounds confusing, but work through it and it makes sense.

Post your results on this thread.

Cheers, Peach

[Right Way Up]

Peachface,
That is virtually long multiplication that you are doing. The 4+3 only works because you are working in numbers less than 20. Seems quite a hard way of doing it. My way of doing it is to split the sum into two parts. i.e. 16 x 17=
10 x 17, & 6 x 17, one very easy part, and one slightly more difficult sum.

[MrBernoulli]

Peachface

It's actually "arithmetic". No offence.

[Paul_atp]

Ummmm.. my head is spinning. I keep a slim pocket sized calculator in my shirt pocket.
In the unlikely event that high G forces prevent me from reaching it.

14 x 13 .... 2(7 x 13)
16 x 17..... 4(4 x 17)
15 x 13 .... 3(5 x 13)
12 x 13 .... (12 x 12) + 13
13 x 13 .... (10 x 13) + (3 x 13)
18 x 51 .... (50 x 18) + 51 .... 10(5 x 18) + 51
68 x 72 .... (60 x 72) + (8 x 72)

OK I admit it, right after 18 x 51 , I crashed !!!

I think we all find a way that suits us in the end :ok:

Regards,

Paul.

[Intruder]

Have a long walk home after work? Try to estimate the mass of the earth in your head, using "back of the envelope" estimates and conversions you already know.

When you come up with an answer, Google a couple places that have figures for it (you'll find they vary a bit).

Can you come within an order of magnitude? I tried a few years ago, when I was working on a Master's degree, and came right between 2 of the figures I found on the net...