Mental Math Tricks Flashcards
6x6
36
Multiples of 9 (up to 10)
Add up to 9, first digit one less than the multiplier
7x6
42
6x8
48
7x7
49
7x8
56
8x8
64
11x_ _ _
=sum of adjacent digits in between digits of original multiplier (unless 1 carries)
Square of 2 digit numbers ending in 5 (35^2)
(first digit) x (n+1), ends in 25 (=1225)
Multiplying two digit numbers beginning with the same digit and last digits add to 10 (84x86)
Answers begin with (first digit) x (n+1), and end with the product of the last digits (=7224)
Multiply any two numbers between 11-19 (18x14)
Add last digit of the second number to the first number, multiply by 10, then add the product of the last two digits (=18+4,=22x10,=220+32=252)
Compliments
First digits (tens, hundreds, thousands, etc.) add to 9, and last (ones place) add to 10 (unless ending in 0)
Adding and subtracting up to three digit numbers
Use rounding/compliments then add or subtract value back at end, go from left to right
Multiplying 2x1 or 3x1
Multiply 100’s, 10’s, and 1’s then add together (use rounding when possible to make problems easier)
Addition Method
Can be used for any multiplication problem but best when one of the numbers ends in a small digit (round down to nearest easy number) 41x17= 17(40+1)= 680 + 17= 697
Subtraction Method
Use when one of the numbers ends in a large digit (round up to nearest easy number then subtract difference) 53x89= 53(90-1)= 4770-53= 4717
Factoring Method
Factor one of the numbers then multiply by each factor. 73x42= 73x7x6= 511x6= 3066 (good because don’t have to mentally hold on to any numbers)
Math of least resistance
Look at problem both ways then take easier path. Try larger factor first but take what is easiest.
Temperature conversion
F=1.8C+32 [1.8=(6x3)/10]
Squaring #s using close together method
Round to nearest multiple of 10, then -/+ the difference in the opposite direction, finally add the square of the difference. 97^2= 100x94 + 3^2= 9400+9= 9409
Multiplying with the close together method
Round one digit to nearest multiple of 10, then -/+ the difference in the opposite direction to the other number, finally add the product of the 2 differences (of original #s to ‘0). *23x26= 20x29 + 3x6= 580+18= 598
- 47x45= 50x42 + (-3)(-5)= 2100+15= 2115
- 43x38= 40x41 + (3)(-2)= 1640-6= 1634
Short Division
Done on paper, faster than long division because do more work in your head and write down less.
Vedic division
Turns a hard two digit division problem into easy one digit division. Remainder moves up one place and add the (result)(multiplier) to the remainders
Major system/ peg system (0-9)
0= s, z/ no peg word 1= t, d/ tea 2= n/ knee 3= m/ moo 4= r/ ear 5= L/ oil 6= ch, sh, j/ shoe 7= k, g/ key 8= f, v/ foe 9=p, b/ pie
More peg words (10-13; 40)
10= dice 11= tot 12= tin 13= tomb 40= rose
Calendar calculator
Month code + date + year code - 7n
Calendar calculator: days
Monday - Saturday = 1- 6
Sunday= 0 or 7
Calendar calculator: months
Jan= 6 (5 leap year) 'winter' Feb= 2 (1 leap year) '2nd month' March= 2 April= 5 May= 0 June= 3 July= 5 Aug= 1 Sept= 4 Oct= 6 Nov= 2 Dec= 4
Calendar calculator: more year codes
1900's= 1 1800's= 3 1700's= 5 1600's= 0 1582= pope Gregory XIII established Gregorian Calendar, universally adopted in 1920's (not always 7 days a week)
Calendar calculator: years 2000-2099
=Years + leaps - 7n
(Leap= divisible by 4 or 400, but not 100)
Repeats every 28 years
