home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
The Human Calculator
/
The Human Calculator.bin
/
hc
/
l2rmul.txt
< prev
next >
Wrap
Text File
|
1993-08-22
|
2KB
|
79 lines
*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 try 36 x 24.
Since this is left to right cross-
multiplication, we start from the
left.
30 x 20 = 600
600 is our base number.$
Now, cross-multiply in two steps:
First, 30 x 4 = 120
Add this to the base number (now
it's 720). Then,
6 x 20 = 120 Add this to
the base number (now it's 840).$
Finally, multiply the last column:
6 x 4 = 24
Add this to the base number to get
the answer: 36 x 24 = 864$
Here's something that helps with
larger numbers. If you multiply
numbers that end in zeros, you can
just multiply the non-zero numbers
and tack on the zeros at the end,
like this:$
Advanced Human Calculators might want
to try some 3-digit numbers now. If
it seems to be getting complicated,
you can skip this part.
Let's try 621 x 584.$
Start on the left:
600 x 500 = 300,000
This is the base number.$
Cross-multiply:
600 x 80 = 48,000
Add this to the base number:
300,000 + 48,000 = 348,000$
Cross-multiply again:
500 x 20 = 10,000
Add this to the 348,000 to get the
new base number: 358,000$
The next cross-multiplication:
600 x 4 = 2,400
Add this to the base number, 358,000
to get the new base number, 360,400$
Multiply:
500 x 1 = 500
360,400 + 500 = 360,900
The new base number is 360,900.$
Next multiply the center digits, 2
and 8. These are tens digits, so
multiply 20 x 80 to get 1600. Add
1600 to the base number of 360,900
to get the new base number: 362,500$
Continuing right, multiply 20 x 4
= 80. Add 80 to the base number. It
becomes 362,580. Then multiply 1 x
80 = 80. Add this to the base number
to get the new one, 362,660.$
Finally, multiply the last digits,
1 x 4 = 4. Add this, and you have the
answer: 621 x 584 = 362,664@