Answer:
36
Step-by-step explanation:
We need to find the LCM of the two numbers because we need to know the lowest number that 12 and 9 are both in.
12 × 1 = 12
12 × 2 = 24
12 × 3 = 36
12 × 4 = 48
9 × 1 = 9
9 × 2 = 18
9 × 3 = 27
9 × 4 = 36
We can see that 36 is the lowest number that occurs both in 12 and 9 so the blue and red light will both link at 36 seconds
