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)#