Answer:
6 seconds
Explanation:
Start by assigning x to value you are trying to find - in this case, the number of seconds until Luigi catches up with Mario.
After x seconds, the distance Luigi has traveled will be the same as the distance Mario has traveled plus 90, since Mario is 90 meters ahead of Luigi.
So our equation is 50x = 35x + 90
Subtracting 35x from both sides: 15x = 90
Dividing both sides by 15: x = 6
So Luigi will catch up with Mario in 6 seconds.