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Maths in a minute: Easy 11

Multiplying $11$ by a positive whole number $a$ that’s less than $10$ is easy. The result is simply $a$ repeated. For example,

$11 \times 2 = 22$

$11 \times 3 = 33$

$11 \times 4 = 44,$

and so on.

Most people know this, but what’s less well-known is that there’s also a neat trick to multiply larger numbers by $11.$ Suppose $a$ is a whole number with two digits. To work out $11 \times a,$ simply work out the sum of the digits of $a$ and drop that sum in-between the digits. For example, let $a = 23.$ The sum of its digits is $2+3=5.$ Dropping that sum between the digits gives $253,$ which is indeed equal to $11 \times 23.$

There’s just one little caveat. If the sum of the digits of $a$ is $10$ or larger, you need to carry a digit. In other words, you stick the right-most digit of the sum between the original digits and add the left-most digit of the sum to the original left-most digit. For example, if $a = 75,$ then the sum of the digits is $12.$ We therefore stick a $2$ between the original digits $7$ and $5$ and add a $1$ to the $7$ to get $825,$ which again is the correct result.

You can convince yourself that this trick always works using long multiplication. Suppose the digits of $a$ are $x$ and $y,$ so $a=xy.$ Long multiplication now tells us that

	$x$	$y$	$\times$	$1$	$1$
$=$			$x$	$y$	$0$
	$+$			$x$	$y$

from which our result easily follows.

Can you work out the trick for numbers $a$ with more than two digits? As a hint, here is the long multiplication when the digits of $a$ are $x_1,$ $x_2,$ $x_3,$ up to $x_ n$ :

	$x_1$	$x_2$	$x_3$	$\dots$	$x_ n$	$\times$	$1$	$1$
=			$x_1$	$x_2$	$x_3$	$\dots$	$x_ n$	$0$
	+			$x_1$	$x_2$	$\dots$	$x_{n-1}$	$x_ n$

Plus.Maths.org

Maths in a minute: Easy 11

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