In one minute, they will blink at the same minute 4 times.
Step-by-step explanation:
The amount it takes for them to blink together is the least common multiple between the time it takes for each of them to blink together.
It takes 3 seconds for the red light to blink and 5 seconds for the blue light to blink. The lcm between 3 and 5 is 15. This means that they blink together each 15 seconds.
A minute has 60 seconds. So in a minute, they are going to blink together 60/15 = 4 times.