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    King K94's Avatar
    King K94 Posts: 1, Reputation: 1
    New Member
    		  
    #1

    Jun 3, 2009, 06:34 PM
    Solving math questions in 5 seconds
    How can I solve 21x29 in 5 seconds?
    Perito's Avatar
    Perito Posts: 3,139, Reputation: 150
    Ultra Member
    		  
    #2

    Jun 3, 2009, 07:45 PM



    Recognize that and that this fits the pattern. Make sure you've memorized all of the squares up to (at least) 25 and can subtract (or add) quickly in your head.
    jcaron2's Avatar
    jcaron2 Posts: 986, Reputation: 204
    Senior Member
    		  
    #3

    Jun 3, 2009, 08:40 PM

    Another way to approach this one easily in your head is to recognize that

    21 x 29 = (20 x 29) + (1 x 29)

    20 x 29 is simply 2 x 29 (which equals 58, of course) with an extra zero at the end (i.e. 20 x 29 = 580)

    Then it's just a matter of adding the two numbers.

    (20 x 29) + (1 x 29) = 580 + 29 = 609
    BlackVY's Avatar
    BlackVY Posts: 823, Reputation: 154
    Senior Member
    		  
    #4

    Jun 3, 2009, 08:47 PM
    Quote Originally Posted by jcaron2 View Post
    Another way to approach this one easily in your head is to recognize that

    21 x 29 = (20 x 29) + (1 x 29)

    20 x 29 is simply 2 x 29 (which equals 58, of course) with an extra zero at the end (i.e. 20 x 29 = 580)

    Then it's just a matter of adding the two numbers.

    (20 x 29) + (1 x 29) = 580 + 29 = 609
    I like this answer... that's the way I'd do it... its easier... :)
    NAIRBP's Avatar
    NAIRBP Posts: 1, Reputation: 1
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    #5

    Jun 4, 2009, 04:27 AM
    Try this approach:

    2 1
    | |
    2 9
    --------
    4 20 9

    =6 0 9

    Step 1: Multiply 1 and 9 =9
    Step 2: Multiply 2 and 2 =4
    Step 3: Cross multiply 2 and 9 = 18
    Step 4: Cross multiply 1 and 2 = 2
    Step 5: Add the answers from step 3 and 4 = 20
    We get number 20 in the step 5, so the 2 is carried over to the left
    and added to 4 which gives 609.

    BPNAIR
    suryasharma's Avatar
    suryasharma Posts: 1, Reputation: 1
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    #6

    Jun 4, 2009, 05:07 AM
    21(30-1)
    630-21
    609
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