The traffic lights at three different road crossings change after every 48 seconds, 72 seconds and 108 seconds respectively. If they all change simultaneously at 8 AM then at what time will they again change simultaneously.

8:09 AM

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8:12 AM

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8:15 AM

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8:18 AM

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Solution

The correct option is B 8:12 AM
Time period after which these lights will change = LCM of 48, 72, 108

48 = 2 × 2 × 2 × 2 × 3
72 = 2 × 2 × 2 × 3 × 3
108 = 2 × 2 × 3 × 3 × 3

LCM (48, 72, 108) = 2 × 2 × 2 × 2 × 3 × 3 × 3 = 432

Let us convert 432 seconds into hours.
432 Seconds = 432÷(60×60)
= 432÷3600
= 0.12 hours

Hence, next simultaneous change will take place at = 8.00 + 0.12 = 8.12 AM

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