Question

The traffic lights at three different road crossings change after every 48 seconds, 72 seconds, and 108 seconds resp. If they change simultaneously at 7 a.m. at what time will they change together again.

Answer 1

Answer:

7 min 12 second

Step-by-step explanation:

48 = 2*2*2*2*3

72 =2*2*2*3*3

108 = 2*2*3*3*3

     LCM = 432

 432 sec = 7 min 12 sec

Answer 2

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If the traffic lights change simultaneously at 8 a.m, then they will change simultaneously again after the LCM of the duration i.e. 48 s, 72 s and 108 s.

LCM of these durations by prime factorisation

48 = 2 × 2 × 2 × 2 × 3 = 2⁴ × 3

72 = 2 × 2 × 2 × 3 × 3 = 2³ × 3²

108 = 2 × 2 × 3 × 3 × 3 = 2² × 3³

LCM of 48, 72 and 108 is 2⁴ × 3³ = 432 seconds.

Hence, They will change after 432 seconds i.e. 7 minutes 12 seconds.

The traffic lights will change after:

8 am + 7 minutes 12 seconds

08 : 07 : 12 am