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Lê Quốc Trần Anh Coordinator

28/05/2018 at 07:24
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How many positive integers in the set of numbers from 1 to 1000, inclusive, are multiples of 2, 3 and 5 but not 8?




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    Huy Toàn 8A (TL) 29/05/2018 at 01:39

    We have : BCNN (2;3;5) = 30

    => BC (2 , 3 ,5) =  \(\left\{30;60;90;120;...\right\}\)

    These numbers are less than 1000

    => BC ( 2, 3 , 5) < 1000 =  \(\left\{30;60;90;120;...;990\right\}\) (1)

    Rally (1) has it all : 

    \(\dfrac{990-30}{30}\) +1 = 33 (numbers)

    Other way , in rally (1) the numbers are B (8) = \(\left\{120;240;...;960\right\}\)  (2)

    Rally (2) has it all : 

    \(\dfrac{960-120}{120}\) = 8 (numbers)

    So has all positive integers in the set of numbers from 1 to 1000 :

    33 - 8 = 25 ( numbers) is a multiple of 2, 3 and 5 but not 8


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