Is there a trick to multiplying two-digit numbers by $11$ ?

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Answer 1 · 2015-09-14T01:50:34

There is a trick you can do with multiplying two-digit numbers by $11$ .

Any two-digit number multiplied by $11$ can be easily found by adding up two digits and placing them in between, adding it on. For example:

$12 * 11 = "1 _ 2"$
$1 + 2 = 3$

$=> 12*11$

$= "1 _ 2"$
$+ "_ 3 _"$
$= color(blue)(132)$

$79 * 11 = "7 _ 9"$
$7 + 9 = 16$

$=> 79 * 11$

$= "7 _ 9"$
$+ "1 6 _"$
$= color(blue)(869)$

$64 * 11 = "6 _ 4"$
$6 + 4 = 10$

$=> 64 * 11$

$= "6 _ 4"$
$+ "1 0 _"$
$= color(blue)(704)$

Is there a trick to multiplying two-digit numbers by 1111?