Answer:
5 cars they need.
Step-by-step explanation:
Given : There are 6 groups of 3 people waiting to go on the underground ride. Each car can take 4 people.
To find : How many cars do you need?
Solution :
There are 6 groups of 3 people waiting to go on the underground ride.
i.e. total number of people are 
Now, In 1 car take 4 people.
1 people =
car
18 people =
car
18 people =
car
Which means 18 people need 5 cars.
Therefore, 5 cars they need.