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Mental Math Multiplication Strategies

**skip counting by numbers 2-12, 25,**

__Strategy #1__and multiples of 10 (ex. 30, 60, 90, 120, etc…)

**using your agenda multiplication chart, take your finger and trace across and down for the two factors, self test without looking at the chart**

__Strategy #2__

__Do the above for 2 – 5 minutes__

**multiplying by 9**

__Strategy #3__a) use your fingers- 2x9= put both hands up, put down your second finger then the tens digit is 1 finger to the left of the finger that is down, and the ones digit is the eight fingers to the right of the finger that is down

**b)**make multiplying by 9 easier by finding ten groups and then taking away one group-ex. 6x9= 6x10-6 15 x 9 = 15x10 -15

15 x 9 = 150-15

15 x 9 = 150 – 10 – 5

15 x 9 = 140 – 5

15 x 9 = 135

19 x 9 = 19 x 10 – 19

19 x 9 = 190 – 10 – 9

19 x 9 = 180 – 9

19 x 9 = 171

MULTIPLYING BY 11: STUDENTS SHOULD EXPLAIN AND DEMONSTRATE THE "11 TRICK"

**multiplying by 25**

__Strategy #4__a) try to break whatever number you are multiplying by 25 into groups of 4 ( reason: 4 x25 = 100)

b) ex. 25 x 12=

25 x 12 = 25 x 4 x 3

25 x 12 = 100 x 3

25 x 12 = 300

c) ex. 25 x 36 =

25 x 36 = 25 x 4 x 9

25 x 36 = 100 x 9

25 x 36 = 900

d) ex. 25 x 29 = this example is a little tougher because 29 is not divisible by 4, so instead try 25 x 28 + one more group of 25 =

25 x 28 + 25 = 25 x 4 x 7 + 25

25 x 28 + 25 = 100 x 7 + 25

25 x 28 + 25 = 725

**Strategy #5**

**multiplying by 99**

· see strategy for multiplying by 9

· find 100 groups of the # you are multiplying by 99 and then subtract 1 group (100x – x)

· ex. 99 x 17 (100 x 17) – 17=

1700 – 17 = 1683

**Strategy #6 multiplying by 19**

See strategy for multiplying by 9, and also use our skills with multiplying by 10 and doubling

Find 10 groups of the # you are multiplying by 19, double it, and then subtract one group

Ex. 19 x 15 = 10 x 15 x 2 = 300 or 20 x 15 = 300

300 – 15 = 285

**Strategy # 7**

**multiplying by 15**

· Find 10 groups of the # you are multiplying by 15 and then add ½ that amount

· Ex. 15 x 24= 10 x 24 =240 240 ÷ 2 = 120

o 240 + 120 = 360

Here are some websites that you might like to use for practicing your mental math skills:

http://www.aplusmath.com/Flashcards/multiplication.html