Numbers and Mental Calculation Strategies | Grade 8 Revision

1 is not a prime number

For a number to be  a prime number it must have only two factors, 1 and itself

First 10 prime numbers: 2,3,5,7,11, 13,17, 19, 23, 29, ...

*9 is odd no., not prime {1,3,9}
*15 is odd no., not prime {1,3,5,15}

#PrimeNumbers #MathFact #MathBuildingBlock

10 common fractions, percentages and decimals equivalents.

½=50%=0.5
⅓=33⅓%=33.3...
¼=25%=0.25
⅕=20%=0.2
⅛=12.5%=0.125
¹/10=10%=0.1
⅔=66⅔%=66.6...
⅖=40%=0.4
¾=75%=0.75
⅗=60%=0.6

#MathSkill #NumberFactsRecall #A_CNumberTag

×12 Recall Tip

12×1=1 2
12×2=2 4
12×3=3 6
12×4=4 8
*12×5=6 0
12×6=7 2
12x7=8 4
12×8=9 6
12×9=10 8
12×10=12 0
12×11=13 2
12×12=14 4

Starting #s increase in 1,2,3...*
Multiples of ×12 end in 0,2,4,6 or 8 and reoccur after 5 #s

Only applies to ×12. #NumberFacts #MathSkillTips

Add 3-digit numbers 

123 + 573
=(100 + 20 + 3) + 500 + 70 + 3
= 100 + 500 + 20 + 70+ 3 + 3
= 600 + 90 + 6
= 696

#Adding

Metal Tips for Division by a decimal 

270 ÷ 0.3
🚦make  0.3 whole # by × 10
I.e. 270 × 10 = 2700; 0.3 ×10= 3

Now, 2700 ÷ 3 = 900

Top tip: In division by decimals, multiply the dividing numbers by 10,  100,... to change decimals to whole #s

#NumberSkills #CoolTips

Dividing a decimal with answer that recurs

E.g. 80÷1.5
🚦×10 make 1.5 whole
80×10=800; 1.5×10=15
--‐‐----------------‐--
🚦simplify
800÷15... ÷5
=160÷3
🚦partition 160...150+10
(150÷3)+(10÷3)
=50+3⅓
=53⅓ (or 53.33...)

*Tip: make decimal whole #; simplify, partition and solve.

Dividing 3-digit numbers

836 ÷ 32
🚦split 836 into easily divisible #s
   |620 | 160 | 32  | 4...sum is 836
 ÷|  32 |   32 | 32  | 32...divisor is 32
   --------‐‐-------------------
    20 +   5  +  1  +  ⅛ = 26⅛ or 25.125

Tip: Go by 20, 10, 5 to reduce #s like 836

#NumberFacts

Mental #multiplication of big numbers

53 × 37
○ 50  | 30
●   3  |   7
-‐-------------------‐--
○50×30=1,500
○50×7=     350
●3×30=       90
●3×7=         21
-------------------------
1,500+350+90+21 = 1961

#MentalMathTips

Long multiplication

53 × 37
------------
     53
   ×37
    '''''''''''''
    371
  159    ....add
  ''''''''''''''''''''
   1961 ...answer
Whichever method is used, have fun🌞

Divide with decimal or fraction

E.g.280 ÷ 3½
🚦Convert fraction to decimal   3½=3.5
Same as 280÷3.5
🚦multiply #s by 10 to make 3.5 whole #
= 2,800÷35
💡35 × 8 = 280
If 35×8=280🔁280÷35=8, so 2800÷35=80
----------------------
So, 280÷3½
=280÷3.5
=2,800÷35
=80

#MentalDivision