Multiplication Trick #2 — Multiply by 15 FAST!

Here’s the second in my set of multiplication tricks. (The first was a trick or multiplying by 5.)

TRICK #2:

WHAT THE TRICK LETS YOU DO: Multiply numbers by 15 — FAST!

HOW YOU DO IT: When multiplying a number by 15, simply multiply the number by 10, then add half.

EXAMPLE: 15 x 6

6 x 10 = 60

Half of 60 is 30.

60 + 30 = 90

That’s the answer: 15 x 6 = 90.

 

ANOTHER EXAMPLE: 15 x 24

24 x 10 = 240

Half of 240 is 120.

240 + 120 = 360

That’s the answer: 15 x 24 = 360.

 

EXAMPLE WITH AN ODD NUMBER: 15 x 9

9 x 10 = 90

Half of 90 is 45.

90 + 45 = 135

That’s the answer: 15 x 9 = 135.

 

EXAMPLE WITH A LARGER ODD NUMBER: 23 x 15

23 x 10 = 230

Half of 230 is 115.

230 + 115 = 345

That’s the answer: 15 x 23 = 345.

PRACTICE Set: (Answers below)

15 x 4

15 x 5

15 x 8

15 x 12

15 x 17

15 x 20

15 x 28

ANSWERS Set:

15 x 4 = 60

15 x 5 = 75

15 x 8 = 120

15 x 12 = 180

15 x 17 = 255

15 x 20 = 300

15 x 28 = 420


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