Gerard Kelly

Short-Cut Math

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    manzoorarshadhas quoted5 years ago
    By adding an amount 10 one number and subtracting an equal amount from a second number, the sum of the two numbers remains the same.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    The 370 − 37 is, of course, 37 × (10 − 1) instead of 37 × 9.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    Either way, the answer is the same.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    To multiply 64 by 25, for instance, you can divide 64 by 4 (= 16) and multiply by 100 (just add two zeros), and there’s your answer: 1,600.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    Take 28 × 15, for instance.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    If you halve 28 (= 14) and double 15 (= 30), you change the problem to 14 × 30 which, as you can see, is an easy 420—the same answer you will get by multiplying 28 by 15.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    If you halve 28 (= 14) and double 15 (= 30), you change the problem to 14 × 30 which, as you can see, is an easy 420—the same answer you will get by multiplying 28 by 15.

    It’s not always the size of a number that makes it difficult to handle but the KIND of number it is. You know, of course, that it’s much easier to multiply by 10 than by 9 even though 10 is the larger number. However, the “difficult” number 9 can be changed into two “easy” numbers, 10 minus 1, which equal 9. Therefore, to multiply a number by 9, you can multiply by 10 (simply add a zero to the number) and subtract the number, thus: 37 × 9 = 370 − 37 = 333.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    They’re inexpensive, easily carried, never need a battery, and give you a record of what you did so the work can be checked.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    For example, to find the sum of 29 and 36, you can add 1 to 29 and subtract 1 from 36 so that the problem becomes 30 + 35; you can see at once that the answer is 65.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    You can apply the same principle to multiplying.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    As you work with short-cuts, you will probably see other ways of calculating faster and easier.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    Discovering methods of your own can add new excitement to your journey through this corner of the magic world of numbers.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    And don’t forget those old reliables, pencil and paper.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    Examples are given under each short-cut to demonstrate how it works.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    The steps are used where needed for additional clarity and are followed by a “Short way” indicating how the method would actually be used.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    If you can do ordinary arithmetic, you will have no trouble in using these methods.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    After that, study whatever methods are of special interest to you.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    The best approach is to concentrate on the methods you can use in your daily work or the ones you would like to learn just for the fun of knowing them.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    This book contains about 150 short-cuts that provide faster, easier ways to add, subtract, multiply and divide.
    Žena Telenor Group Žena Telenor Grupehas quotedlast year
    You can apply them at school, home, shop, or office to save time and trouble.
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