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Thread: find the numbers of boys

  1. #1
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    Lightbulb find the numbers of boys

    The boys of a school can be arranged in 12 15 and 18 equal rows and also into a solid square. What is the least number of boys that school can have? (Hint find the lcm)
    and the answer is 900 but my lcm is 180 and 180 isn't a perfect squred? I know my probpem will be solved!
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  2. #2
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    Re: find the numbers of boys

    No, 180 is not a perfect square! Neither is 2(180)= 360 nor 3(180)= 540, nor 4(180)= 720 but 5(180)= 900 is!

    The point of this problem is that because 180 is the least common multiple, any common multiple of the numbers must be a multiple of 180- and 900 is the smallestmultiple of 180 that is a perfect square.

    A more efficient method of finding that would be to note that 180= 9(20)= 9(4)(5). 9 and 4 are already "perfect squares". To get a perfect square, you need to multiply by 5: 9(4)(25)= 900.
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  3. #3
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    Re: find the numbers of boys

    Thanks hallsoflvy for helping me and i think first method is simplethanks again
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  4. #4
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    Re: find the numbers of boys

    Hello, Zshan96!

    The boys of a school can be arranged in 12, 15, and 18 equal rows and also into a solid square.
    What is the least number of boys that school can have? .(Hint: find the lcm.)

    And the answer is 900 but my lcm is 180, and 180 isn't a perfect square.

    Look at it this way . . .

    . . \begin{array}{ccc} 12 &=& 2^2\cdot 3 \\ 15 &=& 3\cdot 5 \\ 18 &=& 2\cdot 3^2\end{array}

    Hence: . \text{LCM}(12,15,18) \:=\:2^2\cdot3^2\cdot5

    But this is not a square.
    A square has even exponents on each of its prime factors.
    . . And 5=5^1 has an odd exponent.

    Therefore, we need "another 5".
    . . N \;=\;2^3\cdot 3^2\cdot 5^{\color{red}2} \:=\:900
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  5. #5
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    Re: find the numbers of boys

    Thanks soroban but i was taking lcm by common devision thanks brothers
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  6. #6
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    Re: find the numbers of boys

    Thanks soroban but i was taking lcm by common devision thanks brothers another thing i want to ask that my lcm is 180 but why we take lcm as exponent method
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