The answer is 9 trips.
One ferry = 18 cars
You have to divide the number of cars there is by the number of cars the ferry can hold
149 divide by 18 would be around 8. However, when you multiply 18 x 8,, it would be 144 meaning there is five cars left that the ferry need to pick up and so the answer would be 9 since it must take one more trip back.