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Thread: Long Multiplication Help

  1. #1
    Sep 2010

    Long Multiplication Help


    I’m struggling a bit with long multiplication. Every multiplication is going fine, until I hit 48 x 48, which the formula doesn’t seem to be working for. I know this is really simple, but I can’t get my head around why it’s not working. I seem to be able to multiply a hundredth number by a ten, but when it comes to 48 x 48, something’s going wrong.

    Okay, the answer I keep getting is 2324, which isn’t far from the actual answer, which is 2304. I start off by doing 8 x 8, which is 64, so 4 goes in the answer and I carry the 6 over. Then I do 4 x 8, which is 32, I then add the carried over 6, which makes 40, so that goes in the answer, which makes an answer of 404. The correct answer is in fact, 384, so it’s at this point where I’m multiplying by the units that I’m going wrong.

    Now I start by multiplying by the tens, so I firstly put a 0 in the answer as I’m multiplying by tens and not units. Then I start multiplying, firstly 8 x 4, which is 32, so I put 2 in the answer and carry the 3 over. Then I do 4 x 4, which is 16, I then add the carried over 3, which makes 1920. This is the correct answer, so it’s not here that I’m going wrong.

    So with my answers of 404 and 1920, I proceed to add them up. That part is simple, but I won’t go through it as I’ve already gone wrong, which seems to be in the part when multiplying by the units, where I have the answer of 404, but it should be 384.

    I would really appreciate it if you could shed some light as to where I’m going wrong.

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  2. #2
    MHF Contributor
    Prove It's Avatar
    Aug 2008
    $\displaystyle 48 \times 48 = (50 - 2)(50 - 2)$

    $\displaystyle = 50\times 50 + (- 2)\times 50 +(- 2)\times 50 + (-2)\times (-2)$.
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  3. #3
    Super Member
    Jun 2009
    Second line, second paragraph, 32+6=38 not 40.
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