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- When riding my bike in the dark, I use two rear bike lamps. Both of the lamps are in ‘flashing mode’, which means that they repeatedly emit light for a set period of time at set intervals. The reasons for using two lights are that it reduces the amount of time when neither of the lamps emit light, and that if one of the lamps stops working, other road users can still see me on the road. One of the lamps shines for one second and goes dark for one second repeatedly; the other lamp shines for two seconds and goes dark for two seconds repeatedly. I always turn both lamps on simultaneously at the start of my ride. For how many seconds in each minute do neither of the lamps shine when they are in use?
Can you please explain the general formula to answer such questions. I find them quite difficult, they usually come up in each exam.
Thank you 
