The easiest number in the world
to multiply by is !
doesn't change anything! Any
number times is the number itself.
In math when an operation is
done and nothing changes, this is known as an identity. So is the
multiplication identity or the "multiplicative identity", if you want
to be very correct.
Using this identity, we can
grow into the second easiest number on the face of the Earth to multiply
by. That number is !
Now ten is just a one with
a zero attached at the end. So multiplying a whole number by is easy, you just
multiply by and attach a zero at the end. In other words, the number stays
the same except you place a zero at the end.
Ex 1: 10 x 5 = 50
Ex 2: 10 x 27 = 270
Ex 3: 10 x 3.5 = RED ALERT This is not a WHOLE number.
have a decimal portion here.
Now if we place
a zero after the 5 in 3.5, it will look like 3.50 and will not change the VALUE of the number at all!
But this number
needs to be TEN TIMES BIGGER! You see, by placing at the far right of the number AFTER
the decimal point we have added ZERO hundredths or NOTHING.
Have no fear
though, the good part is coming now. The wonderful part about our number
system is that it is BASED on ten numbers only. They are: , , , , , , , , ,and
. Now we can create any number we want with just those digits. The numbers
will be created according to which COLUMN we place the digits.
Now each column
stands for a group of TEN. Each time we add a column before the decimal
point, the number gets TEN TIMES BIGGER.
Let's start with 3.5
If we move that decimal over one column to the right, we have created
35 which is ten times bigger than 3.5
So for short we say that
any time you multiply by TEN, simply move your decimal over one place
to the right.
also works for multiples of TEN like
The number of zeros after the ONE tells you how many decimal places
you should move the decimal TO THE RIGHT.
100 means move two places
1,000 means move three places
10,000 means move four places
(note: remember that all whole numbers have a decimal
point at their rightmost end, EVEN IF YOU DON'T SEE IT.
Ex: 13 can be written 13.,
or 13.0, or even 13.00
Placing zeros after a decimal
point is adding NOTHING to the number. That is why whole numbers are
usually written without their decimal point. It is the number's shortest
form. Math people just assume you know it is there even though you
can't see it. )
Here, let's try a few. Click
on the ? to see if you are correct. *Only works in Internet
- 12.3 x 10 = ?
- 345 x 100 = ?
- 0.45 x 1,000 = ?
- 6 x 10,000 = ?
NOW once you know how to multiply
by TEN and 100 and 1,000 etc.,
you can easily multiply by TWENTY and 200 and 2,000
etc. and by THIRTY and 300 and 3,000 etc.. HERE"S
multiply by TWENTY simply multiply by ten, and then DOUBLE your answer.
This works because twenty is twice as big as ten.
Ex 1a: 14 x 20 = (ten
times 14 = 140; double this to get 280)
people prefer to double the number FIRST, and then multiply by ten.
Ex 1b: 14 x 20 = (double
14 and get 28; then multiply by ten = 280)
will work with 200 and 2000 also. Watch:
Ex 2a: 14 x 200 = (double
14 and get 28; then multiply by 100 = 2,800)
Ex 2b: 14
x 2,000 =(double
14 and get 28; then multiply by 1,000 =
also be done by multiplying by 100 or 1,000 FIRST, then doubling.
Ex 2c: 14
x 200 =(14
times 100 = 1,400; double this to get
Ex 2d: 14
x 2,000 =(14
times 1,000 = 14,000; double this to get
To multiply by THIRTY, take
the same approach but TRIPLE instead
of double your number:
EX 3a: 15 x 30 = (triple
15 to get 45, then multiplying by 10 = 450)
Ex 3b: 15 x 300 =(triple
15 to get 45, then multiply by 100 = 4,500)
Ex 3c: 15 x 3,000 =(triple
15 to get 45, then multiply by 1,000 = 45,000)
as you might have guessed, this approach will work for 40, 50, 60, 70
80, 90 etc. as well.
Here, you try a few. Click
on the ? to see if you are correct. *Only
works in Internet Explorer Browsers
- 13 x 20 = ?
- 2.4 x 200 = ?
- 0.6 x 30 = ?
- 10.5 x 2,000 =?
- 1.25 x 300 = ?
If you got those right, you are
ready to find the treasure!
you KNOW the password
Below are eleven groups
of multiplication problems.
Mentally multiply, and write
down the answer to each problem in each group.
Add the numbers you
get in each group. (You may use paper and pencil to add.)
Look for that sum
in the DECODER BOX.
It will give you a letter of the password.
know the password
23.5 x 100 =
$0.15 x 10 =
4 x 1,000 =
13.5 x 10 =
152 x 100 =
$5.62 x 10 =
0.006 x 10,000 =
1.2 x 1,000 =
13.904 x 100 =
5 x 30 =
15 x 20 =
3.2 x 30 =
$13.50 x 20 =
2.4 x 40 =
0.6 x 30 =
15.2 x 20 =
0.342 x 20 =
1.681 x 20 =
2.34 x 2,000 =
3.2 x 3,000 =
$0.45 x 200 =
31 x 40 =
47 X 200 =
5.8 X 30 =
123 x 30 =
0.0089 x 100 =
5 x 3,000 =
41 x 50 =
84 x 300 =
0.16 x 2,000 =
3.9 x 20 =
7.08 x 200 =
24 x 300 =