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5%
Divide by 10, then divide by 2

15%
Divide by 10, then add 1/2

20%
Divide by 10, then double

Multiplying large numbers (ex. 12 x 640)
10x600 + 10x40 + 2x600 + 2x40

Multiplying by 5
Divide number by 2, then multiply by 10

Percentage as per 100 (ex. 7% of 300, 8% of 250)
 7% of 300 = 7 + 7 + 7 = 21
 8% of 250 = 8 + 8 + 4

Percentages can be flipped (ex. 10% of 50)
10% of 50 = 50% of 10 = 5

Addition from left to right of large numbers (ex. 45 + 34, 459 + 637)
 45 + 34 = (45 + 30) > (75 + 4) = 79
 459 + 637 = (459 + 600) > (1059 + 30) > (1089 + 7) = 1096

