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www.classtopper.com AT A PARTY At a party surprise your friends with this spectacular way of multiplying large numbers together in your head. Here’s how to use VERTICALLY AND CROSS- WISE for multiplying numbers close to 100. � Suppose you want to multiply 88 by 98. Not easy, you might think. But with VERTICAL- LY AND CROSSWISE you can give the answer immediately, using the same method as on the last page. Both 88 and 98 are close to 100. 88 is 12 below 100 and 98 is 2 below 100. You can imagine the sum set out like this: 14
www.classtopper.com 88 - 12 98 - 2 86 24 As before the 86 comes from subtractingcrosswise: 88 - 2 = 86 (or 98 - 12 = 86: you can subtract either way, you will always get the same answer). And the 24 in the answer is just 12 x 2: you multiply vertically. So 88 x 98 = 8624 This is so easy it is just mental arithmetic. Try some: 1) 87 2) 88 3) 77 4) 93 5) 94 6) 64 7) 98 98 –– 97 –– 98 –– 96 –– 92 –– 99 –– 97 –– –– –– –– –– –– –– –– Where there is life there is pattern and where there is pattern there is mathematics JOHN D. BARROW 15
Page 3 and 4: www.classtopper.com On seeing this
Page 5 and 6: www.classtopper.com caught between
Page 7 and 8: 30 CHECK YOUR BILL 32 CHECK YOUR CH
Page 9 and 10: � Similarly 10,000 - 1049 = 8951
Page 11 and 12: � $10 - $2.30 = $7.70 www.classto
Page 13: www.classtopper.com You subtract cr
Page 17 and 18: www.classtopper.com Try a few: 1) 1
Page 19 and 20: So: � 5 + 3 = 20 + 21 = 41 7 4 28
Page 21 and 22: The first part is the first number,
Page 23 and 24: Diagrammatically: 2 x 8 = 16 32 x 3
Page 25 and 26: � Now suppose we want the digit s
Page 27 and 28: www.classtopper.com So all numbers
Page 29 and 30: www.classtopper.com All sums, even
Page 31 and 32: Now suppose that there is a carry f
Page 33 and 34: � Similarly in 73 - 49 we decide
Page 35 and 36: � Similarly 234 x 7 = 1638 We get
Page 37 and 38: ) Multiply crosswise and add: 2x3 +
Page 39 and 40: Practise a few: 1) 21 2) 23 3) 32 4
Page 41 and 42: � So 72 x 11 = 792 Multiply by 11
Page 43 and 44: www.classtopper.com Divide by 9: 1)
Page 45 and 46: Divide these by 9: www.classtopper.
Page 47 and 48: www.classtopper.com To sum up, for
Page 49 and 50: Page 23 1) 2021 2) 624 3) 4216 4) 2
Page 51: www.classtopper.com ANSWERS Page 45

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