Mental Arithmetic (Rules 1-5) Flashcards
Multiplication
What is Rule 1?
If the first digits are the same and the second digits are both 5s then you add 1 to the first digit in the first row and then multiply.
What is Rule 2?
If first digits are the same numbers and the second digits add up to ten then you add 1 to the first digit in the first row and then multiple
What is Rule 3?
If the first digits add up to ten and the second set of digits are the same then you multiply the last row and place the answer, then multiply the first digits and add one of the numbers in the second row of digits
What is rule 4?
Difference of Squares. a^2 - b^2 = (a-b) (a+b) If the difference between two numbers is equal then square the number that is directly between them MINUS how far they are apart^2 SO: (29)(31) = (30 -1)(30+1) 30^2 - 1^2 900 - 1 = 899
What is rule 5?
Any 2-digit number times 11 will equal the first digit in the 2 digit number, the second digit = first digit + second digit of the first two digit number, third digit equals the second digit in the 2 digit number. SO 43 x 11 = 4,(7 (4+3)),3
(45) (65)
2925
(55) (55)
3025
(14) (94)
1316
(24) (84)
2016
(34) (74)
2516
(44) (64)
2816
(13) (93)
1209
(23) (83)
1909
(33) (73)
2409
(43) (63)
2709
(12) (92)
1104
(22) (82)
1804
(32) (72)
2304
(42) (62)
2604
(11) (91)
1001
(21) (81)
1701
(31) (71)
2201
(41) (61)
2501
(51) (51)
2601
