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Four lights light up simultaneously at starting and an interval of 6 sec, 12 sec, 15 sec and 20 sec respectively. How many times do they light up together in 2 hours?

Four lights light up simultaneously at starting and an interval of 6 sec, 12 sec, 15 sec and 20 sec respectively. How many times do they light up together in 2 hours? Correct Answer 121 times 

Given:

Four lights light up simultaneously at starting and an interval of 6 sec, 12 sec, 15 sec and 20 sec respectively.

Concept:

LCM is a multiple of two or more numbers.

Calculation:

LCM of (6, 12, 15, 20) = 60

All 4 lights light up together again after every 60 seconds

Now,

In 2 Hours, they light up together = times + 1 (at the starting) = 121 times

∴ In 2 hours they light up together for 121 times

Mistake Points

In these types of questions, we assume that we have started counting the time after the first light-up. Due to this when we calculate the LCM it gives us the lighting up at 2nd time, not the first time. So, we needed to add 1.

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