Question

two neon lights are turned on at the same time. one blinks every 9 seconds and one blinks every 15 seconds how many seconds will they both blink together

Answer 1

so... if they both blink10 times let's say, the 9 second will be 10 times 9 which is a multiple of nine, the 15 will be a multiple of 15, so you need a number of times x where 9*x=0mod 9 and 15*x=0mod15

Answer 2

ie, it's divisible by both 9 and 15