A green light flashes four times per minute and an red light flashes five times per minute at regular intervals.If both the lights start flashing at the same time,how many times do they flash together per hour?

• Time taken for the green light to flash once = 15 sec. (i.e., 60sec/4times = 15sec)
  Time taken for the red light to flash once = 12 sec. (i.e., 60sec/5times = 12sec)
  
  Time when both of them will flash together = lcm(15,12) = 60 = 1 min.
  
  Hence in an hour they will flash 60 times together. (i.e, 60/1min = 60times)
• 12 years agoHelpfull: Yes(4) No(0)
• when they start together flashing
  Time taken for the green light to flash once = 20sec interval
  Time taken for the red light to flash once = 15 sec.interval
  during the course both together will flash twise in 60 sec.
  hence for 1 hour 120 times
  
  Time when both of them will flash together = lcm(15,12) = 60 = 1 min.
  
  Hence in an hour they will flash 60 times together. (i.e, 60/1min = 60times)
• 12 years agoHelpfull: Yes(0) No(0)
• The green and red lights will start flashing at the same time in the beginning i.e 1 sec.Then they will flash together at 20th,40th and 60th sec.i.e in 1 min they will flash together 4 times.so in an hour it is 60*4=240 times
• 12 years agoHelpfull: Yes(0) No(2)