Final answer:
To find out how many laps Ken and Humid will have done when they reach the start line again, calculate the least common multiple (LCM) of their lap times. This is found to be 240 seconds. Ken will have done 3 laps and Humid will have done 4 laps, for a total of 7 laps.
Step-by-step explanation:
To find out how many laps Ken and Humid will have done when they reach the start line again, we need to find the least common multiple (LCM) of their lap times. Ken takes 80 seconds to run a lap and Humid takes 60 seconds to run a lap.
The LCM of 80 and 60 is 240 seconds (or 4 minutes). This means that Ken and Humid will both be at the start line again after 240 seconds.
Since Ken completes a lap every 80 seconds, he will have done 3 laps (240 divided by 80) when he reaches the start line. Similarly, since Humid completes a lap every 60 seconds, he will have done 4 laps (240 divided by 60). Therefore, Ken and Humid will have done a total of 3 + 4 = 7 laps when they reach the start line again.