Question

Three bulbs red, green and yellow flash at intervals of 80 seconds, 90 seconds and 110 seconds. All three flash together at 8:00 am. At what time will the three bulbs flash altogether again? *
1 point
9 : 00 am
9 : 12 am
10 : 00 am
10 : 12 am​

Answer 1

Three bulbs flash altogether again at 10 : 12 am

Given : Three bulbs red, green and yellow flash at intervals of 80 seconds, 90 seconds and 110 seconds.

All three flash together at 8:00 am.

To find : The time when the three bulbs flash altogether again

9 : 00 am

9 : 12 am

10 : 00 am

10 : 12 am

Solution :

Here it is given that three bulbs red, green and yellow flash at intervals of 80 seconds, 90 seconds and 110 seconds

We now find the LCM of 80 , 90 , 110

80 = 2 × 2 × 2 × 2 × 5

90 = 2 × 3 × 3 × 5

110 = 2 × 5 × 11

LCM

= 2 × 2 × 2 × 2 × 3 × 3 × 5 × 11

= 7920

Now 3600 seconds = 1 hour

∴ 7920 seconds

= 2 hour 720 seconds

= 2 hour 12 minutes

Since all three flash together at 8 : 00 am

Hence the time when the three bulbs flash altogether again

= 8 : 00 am + 2 hour 12 minutes

= 10 : 12 am

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