Mental Math Multiplying Tricks
Tricks to mentally multiply by 1/2.
Somewhere in themathlab.com, locked with a secret password, is a treasure page. This page has over 10 FREE gifts 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 five letters in it.
GOOD LUCK.
Match the password clue letters with the numbers below.
5
3
1
2
4

DECODER GRID
When you KNOW the password


MULTIPLYING BY 1/2 is really nothing more than cutting a number in half. Now sometimes that is an easy thing to do, like 1/2 x 4 = 2, because half of four is two.

Other times it is not quite so easy to cut a number in half, like 1/2 x 735 = ?

Here we will show you some quick and elegant ways to cut in half in your mind.

FIRST:
If all the digits in your number are EVEN, you've got it made. Just divide each digit by two. It doesn't even matter if it has a decimal in it, or if it has a bunch of zeroes. Just CUT EACH DIGIT IN HALF.

Here take a look :

24 x 1/2=12
446 x 1/2 =223
8,426 x 1/2=4,213
6.08 x 1/2=3.04
88,000 x 1/2 =44,000

Now you try a few. Click on the question mark to see if you are correct.

  1. 460 x 1/2 = ?
  2. 4.008 x 1/2 = ?
  3. 0.00624 x 1/2 = ?
  4. 24,000 a 1/2 = ?

SECOND:
If you happen to have a single ODD digit in the number it gets a bit harder to do mentally. Have no fear however, we know a TRICK.

Suppose you have the number 7. Now seven can be thought of as an even number plus one more, or (6 +1). Six is easy to cut in half and so is 1.

So it looks like we need to mentally halve two EASY parts instead of one hard part.

(6 + 1)(1/2) means half of six PLUS half of one, or 3+.5=3.5.

Let's try 9:

Nine = 8 + 1, so we just cut those each in half and put the halves together. Half of eight PLUS half of one is 4+.5=4.5.

What about 90?

That IS an even number because it ends in zero, but it has an ODD digit in it, and therefore it makes our brains slow down a bit when cutting it in half.

Have no fear! This trick works for numbers ending in zeros too.

Think of 90 as (80 + 10). Halve each of those easy numbers and put your results together. Half of eighty PLUS half of ten is 40+5=45.

How about 7,000?

Same idea here. Change the 7,000 to (6,000 + 1,000) and take half of each of those. Half of 6,000 PLUS half of 1,000 is 3,000+500=3,500

THIRD:

Often you will have a mixture of ODD and EVEN digits in a number. Just remember the trick of breaking it up into easy parts, and you will be fine.

Let's take a look at a few examples. We will make them harder as we go. Then we will give you a chance to practice before the password clues are given.

125x1/2=(think, half of 120 is 60 and half of five is 2.5. So the answer is 60+2.5=62.5)

1/2x956=(think, half of 900 is 450, half of fifty is 25, half of six is 3. So the answer is 450+25+3=478)

7.7x1/2=(think, half of seven is 3.5 and half of 0.7 is 0.35. So the answer is 3.85)

YOU TRY THESE:

  1. 455 x 1/2 =?
  2. 67 x 1/2 =?
  3. 890 x 1/2 =?
  4. 0.78 x 1/2 =?

If you got those right, you are ready to try to find the password to the treasure room. Just follow the directions below.


Password Clues
When you KNOW the password

Below are five 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 GRID. It will give you a letter of the password. The groups correspond to the numbers in the boxes at the top.

Group 1:
1/2 x 10 =
1/2 x 0.6 =
14 x 1/2 =
1/2 x 46 =
240 x 1/2 =
68.42 x 1/2 =

Group 2:
5 x 1/2 =
7 x 1/2 =
1/2 x 9 =
30 x 1/2 =
1/2 x 70 =
300 x 1/2 =
1/2 x 5,000 =

Group 3:
1/2 x 23 =
47 x 1/2 =
15 x 1/2 =
1/2 x 29 =
19 x 1/2 =
65 x 1/2 =
8.9 x 1/2 =
101 x 1/2 =

Group 4:
1/2 x 37 =
1/2 x 73 =
95 x 1/2 =
5.5 x 1/2 =
1/2 x 31 =
1/2 x 5.7 =
99 x 1/2 =
75 x 1/2 =
 Group 5:
123 x1/2 =
304 x 1/2 =
1/2 x 750 =
1/2 x 943 =
552 x 1/2 =
397 x 1/2 =
5,690 x 1/2 =

DECODER GRID

153.95
192.21
4,379.5
2,710.5
7,480
189.51
210.6
2,711.5
o
f
m
e
s
n
y
r

 

When you know the password

 

To learn even more of these mental math tricks try the book cover.

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