Hello, Juliette,
Since there are 72 cars in a parking lot consisting of 108 cars and trucks, we can subtract the 72 from the total to find the number of trucks.
108 - 72 = 36 trucks
The ratio of trucks to cars becomes (36 trucks/72 cars). This can be reduced to 1/2.
The ratio of trucks to cars is 1/2 or 1:2.
Bob