Mental Math Multiplying Tricks
Tricks to mentally multiply by 5, 25, and 50 etc..

Somewhere in themathlab.com, locked with a secret password, is a treasure page. This page has funky surprise for you.

You are now looking at the TREASURE FINDER page that will lead you to this password.

Read through the page below and do what it says. Write down any parts of the password as you find them.

You will need paper and pencil to keep track of your mental math results.

This treasure page's password has twelve letters in it, arranged in the order you see below. GOOD LUCK.
12
6
2
9
5
11
1
7
4
8
3
10

When you KNOW the password

TRICK FOR MULTIPLYING BY 5:

A very important thing to remember is that any number can be expressed in many ways. For example, 5 can be thought of as 10x1/2 or as 1/2x10.

As we know, it's very easy to multiply any number times 10 using the TEN TRICK, and to multiply by 1/2 using the HALF TRICK.

So we simply think of multiplying a number by 5 as a "combination" operation. We move the decimal one spot to the right and cut this new number in half, OR we cut the number in half first and then move the decimal one spot to the right.

Here let's look at a few:

24 x 5 =

(Think: 24 times 10 is 240, and half of 240 is 120.
So 24 x 5 = 120. OR if you prefer, think: 24 times 1/2 is 12, and then 12 times 10 is 120.)
*Sometimes it's easier to cut in half FIRST, and sometimes multiplying by 10 FIRST is easier. You decide.)

3.8 x 5 =

(Think: move the decimal right one spot, get 38, then cut this in half; answer...19)

2,462 x 5 =

(Think: cut in half, get 1,231, and move decimal one spot right; answer...12,310)

TRICK FOR 50:

A slight modification of the FIVE TRICK will work for 50 and for 500 etc.. You simply remember that
50 = 100 x 1/2 OR 1/2 x 100
500 = 1,000 x 1/2 OR 1/2 x 1,000

Look at these :

26 x 50 =

(Think: half of 26 is 13 and 13 times 100 is 1,300. So the answer is 1,300)

50 x 0.45 =

(Think: 0.45 times 100 is 45 and half of 45 is 22.5)
*We thought it was easier to multiply by 100 FIRST on this one, because it got rid of the decimal before we had to cut in half.

2.82 x 500 =

(Think: 2.82 x 1,000 = 2,800 and half of that is 1,410)

You try these. Click on the ? to see if you are correct.
*This only works in Internet Explorer.

  1. 50 x 0.32 = ?
  2. 4.9 x 5 = ?
  3. 37 x 500 = ?

THE TRICK FOR MULTIPLYING BY 25:

This trick is very similar to the trick for 5 and 50. It depends upon you knowing that 25 = 1/4 x 100, or put another way, it is half of half of 100. (1/2x1/2x100). *Remember 1/4 = 1/2 x 1/2.

So to multiply by 25, we simply cut the number in half TWICE (or divide my four) and move the decimal point 2 spots to the right. Or vice versa if you prefer.

Look at these two examples :

25 x 284 =

(Think: half of 284 is 142 and half of that is 71. Now multiply 71 times 100 to get 7,100) *You get the same result by mentally dividing 284 by four and then multiplying by 100.

25 x 0.96 =

(Think: 100 times 0.96 = 96, and half of 96 is 48, and half of that is 24) *Again either cut in half TWICE or mentally divide by 4 whichever is easier for you.

Give these a try now. Click on the ? to see if you are correct. *This only works in Internet Explorer.

If you get them correct, you are ready to find the Password Clues.

  1. 25 x 16 = ?
  2. 25 x 36 = ?
  3. 50 x 41 = ?
  4. 5 x 19 = ?
  5. 2.6 x 500 = ?

Password Clues

Below are twelve multiplication problems.
Mentally multiply, and write down the answer to each.

Look for each answer in the DECODER GRID. It will give you a letter of the password.

When you know the password
    12
    6
    2
    9
    5
    11
    1
    7
    4
    8
    3
    10

 

1.) 32 x 5 =
2.) 25 x 88 =
3.) 50 x 128 =
4.) 0.82 x 50 =
5.) 5 x 123 =
6.) 124 x 25 =
7.) 50 x 16.89 =
8.) 0.49 x 5 =.
9.) 25 x 0.36 =
10.) 7.2 x 50 =
11.) 96.4 x 25 =
12.) 500 x 3.2 =

Decoder Grid

9
2,200
41
241
1600
844.5
615
2.45
z
z
e
c
p
r
l
n
90
160
3,100
360
2,410
36
8,449
6,400
d
f
u
y
e
b
t
z

 



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