Selection round 2019 Mymensingh Higher Secondary #5

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samiul_samin
Posts:1007
Joined:Sat Dec 09, 2017 1:32 pm
Selection round 2019 Mymensingh Higher Secondary #5

Unread post by samiul_samin » Mon Jan 21, 2019 11:36 pm

Four bells are ringing respectively after $2$,$3$,$5$,$6$ hours.Before ringing together how many bell-ring will be heard by you?You cannot distint two or more bells when they ring together.So multiple bell-ring at the same time will be counted as $1$ bell-ring.

Note
Sorry for grammatical mistakes.

samiul_samin
Posts:1007
Joined:Sat Dec 09, 2017 1:32 pm

Re: Selection round 2019 Mymensingh Higher Secondary #5

Unread post by samiul_samin » Mon Jan 21, 2019 11:47 pm

Answer
$21$
Solution
Complementary problem: How many numbers are prime but greater than $6$ and less than $30$.
:D :) :) :D ;)
Just write them down and strike of the prime numbers:
Prime numbers are $7,11,13,17,19,23,29$
So answer is $28-7=21$

This problem can also be solve by using
PIE
or
By calculating multiple LCM

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