Mental Math Multiplying Tricks

Tricks to mentally multiply 15, 1.5, and 15%

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.

The trick for multiplying by 15 is a combination trick. It involves using the trick for 10 and the trick for 5. If you are not skilled at those tricks, go learn how to use them before you attempt the 15 trick.

Okay, now that you have the required skills, here we go.

Let's try the problem 24 x 15:

FIRST:

Remember that 15 can be written as (10 + 5) so 24 is really being multiplied by BOTH PARTS, 10 & 5.
Think of the problem as two EASY multiplication problems added together. 24(10 + 5) = (24x10) + (24x5)

SECOND:

Take 24 x 10 and get 240 in your head.

THIRD:

Take 24 x 5 and get 120
(*Remember, once you have 24x10=240, just cut this in half to get 24x5 = 120)

FOURTH:

Now add 240 and 120 to get 360.

*All four steps can be done mentally if you like.

Here, you try a few. Click on the ? to check your answers.
*? answers only work in Internet Explorer.

  1. 15 x 8 = ?
  2. 30 x 15 = ?
  3. 6.2 x 15 = ?
  4. 48.6 x 15 = ?

*NOTE:

You can multiply by 1.5 similarly. Just multiply by 1 and also by 1/2 (which = 0.5), then add the results together.

EX: 48.6 x 1.5 =

Think: (48.6 x 1) + (48.6 x 1/2) which gives
+ 48.6
+ 24.3
--------
+ 72.9

*NOTE:

Often in restaurants you like to leave a 15% tip. This can be easily figured out if you remember that 10% + 5% = 15%. So find 10% of the bill, then find 5% of the bill, by cutting the 10% amount in half, then add these results together.

EX: $48.60 x 15% =

Think: 10% of $48.60 is $4.86 (Remember to find 10% of any number just move the decimal 1 spot to the LEFT.)

Now half of $4.86 is $2.43. So adding we get,
+$4.86
+$2.43
---------
+$7.29

You try these. Click the ? to check.

  1. 1.5 x 72 = ?
  2. 24 x 1.5 = ?

  3. $16.80 x 15% = ?

  4. $6.42 x 15% = ?

Password Clues

As you use the 15 tricks to do the following problems IN ORDER, write down the letter which corresponds to each answer.

When you KNOW the password

  1. 15 x 6 =
  2. 15 x 14 =
  3. 16 x 15 =
  4. 15 x 15 =
  5. 30 x 15 =
  6. 1.5 x 20 =
  7. 90 x 1.5 =
  8. 15% x $1.60 =
  9. $12.50 x 15% =
  10. 15 x 2.6 =
  11. 1.5 x 13 =
  12. 120 x 1.5 =
  13. 4.82 x 15 =
  14. $50.62 x 15% =
  15. 15% x $29 =
  16. 1.5 x 43 =
  17. 15 x 17 =
  18. 38 x 1.5 =
  19. 15% x $12.82 =
  20. $15 x 15% =
  21. Now cross off every a,b,c,and f. You should now be looking at the correct password.
  • 210 = g
  • $1.88 = s
  • 225 = a
  • $4.35 = o
  • 240 = r
  • 450 = a
  • 135 = b
  • 64.5 =c
  • $1.92 = b
  • 57 = b
  • $38 = h
  • 255 = u
  • 90 = a
  • 30 = o
  • 39 = b
  • $0.24 = f
  • $2.25 = t
  • 180 = f
  • $7.59 = c
  • 19.5 = s
  • 72.3 = f
  • $2.50 = r

When you KNOW the password

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

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