Yara's
Finger-math Subtraction Trick
Yara Smilde came up
with a cool finger-math trick for subtraction. It
doesn't work for all subtraction
problems. However, it works for those hard subtraction problems in which the
“ones” digit of the top number (the “minuend”) is
smaller than the ones digit of the bottom number (the “subtrahend”).
Basic
operation
Let´s
say you need to solve the following subtraction problem:
17
-8
1.
You start with your two hands closed.
2.
Then you take the number on the bottom, 8, and lift that many fingers up
3.
You lower as many fingers as the number in the ones column of the minuend, in
other words 7. This leaves you with 1 finger up.
4.
You lower the fingers that are up, and raise the fingers that are down and you
end up with nine which is the answer (the “difference”).
It
takes a little practice to get used to, but once you have it down, it can be
done really fast.
Larger numbers
If
the minuend is 20 or larger, it can be organized like a regular math problem, for
example
52
-28
First
you subtract one from the tens digit in the minuend and then subtract the tens
digit of the subtrahend from this, in other words 4 – 2 = 2 in the tens
column. Then you proceed as described above. From closed hands, raise eight
fingers, lower 2 which leaves six. Lower those six
that are up as you raise the four that were down and that leaves 4 for the ones
column. The result is 24.
Let Yara know if it works for you!
dsmilde@yahoo.com
Yara Smilde is a 4th grader at the
Centro Educativo de la Associación de
Profesores de la Universidad Central de Venezuela (CEAPUCV) in Caracas. She
first started doing finger math tricks with her second grade teacher Heather Sitler at
February 2008