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Q.

Three alarm clocks ring at intervals of 6,9 and 15 minutes respectively. If they start ringing together, after what time will they next ring together?


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a

    120 minutes

b

    90 minutes

c

    110 minutes

d

    60 minutes 

answer is B.

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Detailed Solution

Given,
Three alarm clocks ring at intervals of 6, 9 and 15 minutes respectively.
The LCM of 6, 9 and 15 are,
⇒6=2×3   ⇒9= 3 2  
  ⇒15=3×5   ⇒LCM(6,9,15)=2× 3 2 ×5 ⇒LCM=90  
Thus, after 90 minutes all three alarms will ring together.
Therefore, option(2)     is correct answer.
 
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