Mental Math Multiplying Tricks
Tricks to mentally multiply 2, 4, and 8
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 eleven letters in it. GOOD LUCK.

When you KNOW the password

MULTIPLYING BY TWO is just doubling. Simply add the number to itself. If the number is long, you can double it in place value pieces.

EX: 753 x 2 = (700+50+3)2 or
Think:
+ 700+50+3
+. 700+50+3
-----------------
1400+100+6 = 1506
*We like to add these from LEFT to RIGHT, keeping a running total in our heads. 1400+100=1500 and 1500+6=1506.

HERE try a few for yourself. Click the ? to see if you are correct.
*The answer check ?'s only work in Internet Explorer.

  1. 859 x 2 = ?
  2. 427 x 2 = ?
  3. 3,941 x 2 = ?
  4. 3.43 x 2 = ? *Don't forget to keep the decimal in the same spot.

MULTIPLYING BY FOUR is simply multiplying by two TWICE. You just double the number TWICE, because 4=2x2.

EX: 32 x 4 =
Think: 32 x 2 = 64 and 64 x 2 = 128

EX: 0.45 x 4 =
Think: double 0.45 to get 0.9; double 0.9 to get 1.8

MULTIPLYING BY EIGHT is the same as doubling THREE TIMES, because 8=2x2x2.

This trick may take a bit of practice before your speed gets fast.

EX: 27 x 8 =
Think:
double 27 get 54
double 54 get 108
double 108 get 216

EX: 5.2 x 8 =
Think:
double 5.2 get 10.4
double 10.4 get 20.8
double 20.8 get 41.6

EX: 639 x 8 =
Think:
double 639 get 1278, then double and get 2556, then double and get 5112.

*It's fun to challenge yourself to multiply by eight like this. Remember, you can always check your answer by casting out nines.

Password Clues

Use the tricks for 2, 4, and 8 to find the answers to each problem below IN ORDER. (Challenge yourself to do them all mentally) Look for the answer in the right hand column and write down the letter that corresponds to it.

When you KNOW the password

 
  1. 2 x 17 =
  2. 13 x 4 =
  3. 59 x 2 =
  4. 123 x 4 =
  5. 2 x 13.2 =
  6. 8 x 14 =
  7. 4 x 0.32 =
  8. 960 x 2 =
  9. 15 x 8 =
  10. 4 x 16.34 =
  11. 3.41 x 2 =
  12. 2 x 57.7 =
  13. 8 x 719 =
  14. 37.08 x 4 =
  15. 5.38 x 2 =
  16. 4 x 33 =
  17. 2 x 5,678 =
  18. 0.15 x 4 =
  19. 1.12 x 8 =
  20. 29 x 2 =
  21. Now cross off every c, r, and t. You should then see the password.
  • 65.36 = k
  • 10.76 = b
  • 26.4 = o
  • 58 = s
  • 34 = c
  • 492 = r
  • 1.28 = t
  • 11,356 = c
  • 5,752 = e
  • 1,920 = c
  • 0.6 = l
  • 6.0 = t
  • 6.82 = r
  • 115.4 = c
  • 118 = m
  • 54 = r
  • 8.96 = l
  • 132 = a
  • 148.32 = y
  • 52 = c
  • 112 = n
  • 120 = r
  • 11,346 = t

 

When you KNOW the password

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