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r2lmul.txt
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1993-08-13
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*Cross-multiplication allows you to
solve amazingly large multiplication
problems in your head. Both methods,
right to left and left to right, work
just as well, so let's try them to
see which one you like better.$
Let's multiply 14 by 12 using right
to left cross-multiplication.$
To find the last digit in the
answer, multiply the two numbers on
the right:
2 x 4 = 8$
To find the middle digit, we cross-
multiply two sets of numbers and add
the results.
2 x 1 = 2
4 x 1 = 4
Add the 2 and 4. 2 + 4 = 6$
To find the left part of the answer,
just multiply the leftmost digits.
Since:
1 x 1 = 1
the answer is 168.$
This strategy gets even easier with
practice. Let's look at a problem
with larger numbers and see what
happens. We'll try 62 x 12 using
right to left cross-multiplication.$
Start on the right:
2 x 2 = 4
The last digit in the answer is 4.$
Now cross-multiply, and add the
results: 2 x 1 = 2, and 2 x 6 = 12.
12 + 2 = 14. The 4 is the middle
digit. Carry the 1.$
The last step is to multiply the
numbers on the left:
6 x 1 = 6
Add to this the 1 we carried. The
left part of the answer is 7.
The answer is: 62 x 12 = 744$
If you like, you can skip the next
part, but if you want to see how
the Human Calculator does 3-digit
problems with right to left cross-
multiplication, let's try 621 x 584.$
O.K. Let's go! Start on the right:
1 x 4 = 4 4 is the last digit
in the answer. Now cross-multiply:
4 x 2 = 8 and 1 x 8 = 8
Add the results: 8 + 8 = 16
Put down the 6, and carry the 1.$
Cross-multiply:
4 x 6 = 24
1 x 5 = 5
and 2 x 8 = 16
Add all the results together. Add
the 1 we carried. We get 46.$
Next: 8 x 6 = 48
and 2 x 5 = 10
Add: 48 + 10 = 58
Add the 4 we carried from the
previous step:
58 + 4 = 62$
Last, multiply the leftmost digits:
6 x 5 = 30
Add the carried 6 from the previous
step: 30 + 6 = 36
Done! 621 x 584 = 362,664@