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Mental Math Multiplication Strategies
Strategy #1 skip counting by numbers 2-12, 25,
and multiples of 10 (ex. 30, 60, 90, 120, etc…)
Strategy #2 using your agenda multiplication chart, take your finger and trace across and down for the two factors, self test without looking at the chart
Do the above for 2 – 5 minutes
Strategy #3 multiplying by 9
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"
Strategy #4 multiplying by 25
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
and multiples of 10 (ex. 30, 60, 90, 120, etc…)
Strategy #2 using your agenda multiplication chart, take your finger and trace across and down for the two factors, self test without looking at the chart
Do the above for 2 – 5 minutes
Strategy #3 multiplying by 9
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"
Strategy #4 multiplying by 25
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