Posers and Puzzles

Posers and Puzzles

  1. Joined
    15 Oct '07
    Moves
    4056
    30 Jul '08 09:51
    so im doing a few maths questions in preparation for this maths comp we have tommorow, and one question that was seemingly obvious had me stumped with the answer.

    more or less, its ends up being a question of 7.6 - 16.7
    easy yeah?

    if u go thru the whole ordeal of setting up the subtraction table the answer wuld be -10.9 rite?

    7.6
    - 16.7
    = -10.9

    the actual answer: -9.1
    WTH?

    is there some rule that you cant subtract like this or something cuz if there is, i neva learnt it
  2. Standard memberPalynka
    Upward Spiral
    Halfway
    Joined
    02 Aug '04
    Moves
    8702
    30 Jul '08 10:27
    Originally posted by banx99
    so im doing a few maths questions in preparation for this maths comp we have tommorow, and one question that was seemingly obvious had me stumped with the answer.

    more or less, its ends up being a question of 7.6 - 16.7
    easy yeah?

    if u go thru the whole ordeal of setting up the subtraction table the answer wuld be -10.9 rite?

    7.6
    - 16.7
    = ...[text shortened]... there some rule that you cant subtract like this or something cuz if there is, i neva learnt it
    If you want to use the subtraction table, then use the following property:

    7.6 - 16.7 = -(16.7-7.6)

    So just calculate 16.7 - 7.6 and add a minus sign in the end.
  3. Joined
    12 Sep '07
    Moves
    2668
    31 Jul '08 07:11
    Haha! That's exactly what i told you banx99, and you weren't satisfied. Well, there you go! Oops too late the competitions over. XD
  4. Standard memberwolfgang59
    Mr. Wolf
    at home
    Joined
    09 Jun '07
    Moves
    46043
    04 Aug '08 08:06
    Originally posted by banx99
    so im doing a few maths questions in preparation for this maths comp we have tommorow, and one question that was seemingly obvious had me stumped with the answer.

    more or less, its ends up being a question of 7.6 - 16.7
    easy yeah?

    if u go thru the whole ordeal of setting up the subtraction table the answer wuld be -10.9 rite?

    7.6
    - 16.7
    = ...[text shortened]... there some rule that you cant subtract like this or something cuz if there is, i neva learnt it
    A good example of why learning algorithms is bad practice unless you understand the mechanism behind it!

    With the subtraction algorithm you subtract .7 from .6 by 'borrowing' 1.0. That gives 1.6 - 0.7 = 0.9. But what does that 0.9 mean? It is POSITIVE 0.9.

    Now the rest of the subtraction is 6 - 19 which gives NEGATIVE 10.

    So the answer is the sum of NEGATIVE 10 and POSITIVE 0.9 which is as you found out is -9.1

    Of course nobody would do it this way but it does work!
  5. Joined
    18 Jul '06
    Moves
    18228
    05 Aug '08 01:173 edits
    Originally posted by banx99
    [b]so im doing a few maths questions in preparation for this maths comp we have tommorow.
    Use a number line, the number line never lies.
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