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Math Help - number of integers bet 90 and 990

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    rcs
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    number of integers bet 90 and 990

    How many integers bet 90 and 990 are divisible by 7?

    is there any short and simple way of finding the answer?

    thanks
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    Re: number of integers bet 90 and 990

    Quote Originally Posted by rcs View Post
    How many integers bet 90 and 990 are divisible by 7?

    is there any short and simple way of finding the answer?

    \left\lfloor {\frac{{990}}{7}} \right\rfloor  - \left\lfloor {\frac{{90}}{7}} \right\rfloor=~?
    Thanks from rcs
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    Re: number of integers bet 90 and 990

    Quote Originally Posted by rcs View Post
    How many integers bet 90 and 990 are divisible by 7?

    is there any short and simple way of finding the answer?

    thanks
    Hi rcs!

    The answer is 990 / 7 - (90 - 1) / 7, where "/" denotes a division that is rounded down.
    I am assuming that you would count 990 and 90 if they would have been divisible by 7 (which they are not anyway).

    To explain:
    (90 - 1) / 7 is the number of integers divisible by 7 below and excluding 90.
    990 / 7 is the number of integers divisible by 7 below and including 990.
    The difference is the total number.

    Suppose you are in the middle of a road with poles alongside the road.
    And suppose you want to know the number of poles from where you are to the end of the road.

    To find out you count the number that you can see from where you stand to the beginning of the road.
    And if you are next to a pole you do not count that one.
    Subtract from the total number of poles and there you are.
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    Re: number of integers bet 90 and 990

    thanks Prof . Plato
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    Re: number of integers bet 90 and 990

    Quote Originally Posted by Plato View Post
    \left\lfloor {\frac{{990}}{7}} \right\rfloor  - \left\lfloor {\frac{{90}}{7}} \right\rfloor=~?
    the answer is 128 4/7 is this correct sir?

    i wonder why the book the answer they posted is 56... however they do not post the solution
    Last edited by rcs; December 22nd 2012 at 07:14 AM.
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    Re: number of integers bet 90 and 990

    Quote Originally Posted by rcs View Post
    the answer is 128 4/7 is this correct sir?
    i wonder why the book the answer they posted is 56... however they do not post the solution
    First that is the floor function. It returns an integer.

    There are \left\lfloor {\frac{{990}}{7}} \right\rfloor=141 integers from 1 to 990 that are by seven.

    There are \left\lfloor {\frac{{90}}{7}} \right\rfloor=12 integers from 1 to 90 that are by seven.

    Note that neither 990 nor 90 is divisible by seven.

    So you book is not correct. The answer is 129.
    Last edited by Plato; December 22nd 2012 at 07:49 AM.
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    Re: number of integers bet 90 and 990

    Quote Originally Posted by Plato View Post
    First that is the floor function. It returns an integer.

    There are \left\lfloor {\frac{{990}}{7}} \right\rfloor=141 integers from 1 to 990 that are by seven.

    There are \left\lfloor {\frac{{90}}{7}} \right\rfloor=12 integers from 1 to 90 that are by seven.

    Note that neither 990 nor 90 is divisible by seven.

    So you book is correct. The answer is 129.
    you mean to say that the book i've read whose answer is 56 is correct and your answer 129 is also correct... therefore, there are two correct answers now... 56 and 129
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    Re: number of integers bet 90 and 990

    Sorry that was a typo. It should be NOT CORRECT
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    Re: number of integers bet 90 and 990

    ahhh i see... well then thank you again Sir. God Bless
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    Re: number of integers bet 90 and 990

    Suggest we find the multiple of 7 just greater than 90 and just less than 990.
    We will get 91 and 987. Now all the multiples of 7 between these two numbers will form an AP with first term 91, common difference 7 and last term 987.
    Question now simply is to find n such that the nth term is 987.
    If a is the first term, common difference d then the nth term is given by
    an = a + ( n-1) d
    Thus we have 91 + ( n 1 ) x 7 = 987
    OR 91 + 7n 7 = 987
    7n = 987 91 + 7 = 903
    n = 129 Hence there are 129 integers between 90 and 990 which are divisible by 7
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