A Quick Method to Multiplying 2 Digit Factors
Up to 20
Investigation # 7
Submitted by: Ethel L. Jones
In 3rd and 4th grades there must be constant efforts to
allow the students ways to memorize and understand
multiplication and division facts. The mastery of
division begins with a firm grasp of multiplication. It is
a struggle for children to retain multiplication facts.
Therefore, it is important that they be given alternative
methods to arrive at answers.
The particular shortcut will benefit both, the weak
student as well as the more able student. It will review
addition facts while working on multiplication facts of
factors up to 20. It will also reinforce skills in the area
of place value.
The author, Vic Evans, wrote to always make sure that
the bigger numbers were on top. It was observed in this
investigation that as in any other multiplication process,
this did not seem to make a difference.
However, for the sake of any error, the directions were
carried out in multiplying any two numbers from 11to
19 without the use of a calculator:
http:www.angelfire.com/me/marmalade/mathtips.html
Examples:
Take 15 X 13
Cover the 15 and the 3. These covered numbers are all
that will be used.
Add 15 + 3
Add a zero to that sum. ( Multiply it by 10)
Multiply the lower number ( 3) by the number directly
above it (5).
Add the two numbers together. (180 + 15) . That is the
answer.
15
X13
15+ 3 =18
5X 3 = 15
10 X 18 = 180
Add
15
195
Check:
15
X13
45
15
195
17
X 15
17 + 5 = 22 X 10 = 2 2 0
5 X7 = 35
35
255
Check:
17
X 15
85
17
2 55