Rip it Apart

Welcome to the
Rip it Apart Trick
a.k.a. the distributive trick

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.

Of all the mental math tricks in themathlab.com POWERZONE, we feel this trick is the most powerful, because we use it the most. In fact, this trick can be used in place of many of the other tricks. If you only want to learn one trick, this is the one to learn. So here goes.

The basis of this trick is the process of ripping apart a number into its simpler parts and then using those simple parts instead of the whole number all together.

For example: Multiplying by 37 mentally is a bit difficult, so we rip it apart into 30 + 7. Now it's easy to multiply by 30 and also by 7. Then we simply add the two products together.
*(Technically, this is known as the distributive property of multiplication over addition)

EX: 37 x 5 = (30+7)5
Think:
30x5=150
+7x5 = 35
------------
ANSWER = 185

EX: 49 x 7 = (50-1)7
*NOTE: here we chose to use (50-1) instead of (40+9). This is a personal preference. We think it's easier to multiply by 50 and by 1 and then subtract the products. This is also known as the nine trick: answer 350-7=343.

EX: 127 x 6 = (100+20+7)6
Think in your head (6x100) + (6x20) + (6x7)
600
120
+42
-----
762

EX: 3.8 x 5 = (3+0.8)5
Think: 15 + 4 = 19

or if you prefer another approach

Think: 3.8 x 5 = (4-0.2)5
20 - 1 = 19
*Some folks prefer to think of 3.8 as (4-0.2), because they'd rather multiply by .2 than by 8.

Password Clues Use the "rip it apart" trick to do all the problems on the left IN ORDER. As you work, find the corresponding letter for each answer on the right, and write them all down IN ORDER. When you KNOW the password
3 x 27 = 52 x 4 = 19 x 7 = 123 x 3 = 12 x 8 = 34 x 5 = 13 x 7 = 252 x 3 = 14 x 9 = 72 x 6 = 16 x 7 = 391 x 4 = 24 x 8 = 713 x 5 = 17 x 5 = 824 x 6 = 15 x 4 = 11 x 15 = 3 x 18 = 8 x 17 = Now cross off every c, i, h, b, and f. You will be left with the password for the treasure room.	91 = f 81 = n 82 = c 1,564 = e 133 = i 432 = r 208 = h 54 = a 96 = t 170 = u 60 = e 126 = f 165 = b 136 = k 155 = f 192 = t 112 = c 3,565 = b 85 = h 4,944 = w 756 = i 369 = a

When you KNOW the password

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

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