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.