ANSWER:
30 cars
Explanation:
The first thing is to calculate the total cost for 4 people, like this:

Now, we must subtract the budgeted value, to know the amount they need to complete the total cost of the trip, like this:

The four students decided to earn the additional amount by washing cars.
They charge $6 for each car wash. Therefore, the number of cars to get $175 would be:

Because if they wash 29 cars, they will not be able to reach the necessary amount, they must wash at least 30 cars