Two neon signs are turned on at the same time. both signs blink as they are turned on. one sign blinks every 9 second's. the other every 15 seconds. in how many seconds will they blink together again

For this question you have to find the lowest common multiple of 9 and 15. there are a few ways to do that. in this case the lowest common multiple (LCM) is 45. this means that both lights will blink at the same time again after 45 seconds. 

It is  standard formthe one u wrote