# lcm 英文 文字題

there are 3 sets of flashing lights. all of the lights are off at 1:00. the first set will remain on for 20 seconds then off for one second. the second set will remain on for 15 seconds then off for one second. the third set will remain on for 6 seconds then off for one second. how many seconds after 1:00 will all 3 sets of lights be off again for the first time?

Rating
• ?
Lv 7

To find the time for all 3 sets of lights be off again, the LCM need to be find.

Because the lights are off for one second, it needs to add one second for each period.

The period of the first set is 21seconds. (on for 20s and off for 1s)

________on___________________on___________________on___________

The period of the first set is 16seconds. (on for 15s and off for 1s)

________on______________on______________on______

The period of the first set is 7seconds. (on for 6s and off for 1s)

__on_____on_____on_____on_____on_____on_____on_____on___

The LCM of 21, 16 and 7 is : 336

All the lights finished off period after 336s

Since the off period is 1s, all the lights are off for the first time after 335 seconds.