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Joe was in charge of lights for a dance. The red light blinks every two seconds, the yellow light every three seconds, and the blue light every five seconds. If we include the very beginning and very end of the dance, how many times during a seven minute dance will all the lights come on at the same time? (Assume that all three lights blink simultaneously at the very beginning of the dance.)

 Apr 1, 2019
 #1
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LCM (least common multiple) of 2  3  and 5  is   30 seconds.....so each 30 seconds 2 times per minute), all of the lights will be on

   ......including the initial 'all on' time of zero seconds....that would be 15 times in 7 minutes.

 Apr 1, 2019

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