There's an easy way to solve this.
The ratio is 7:3 as cars:trucks
The difference is that it starts off with 4 more cars than trucks.
We want to find how many of each are there when there are 16 more cars than trucks.
16÷4
4
Let's multiply each side of the ratio by 4.
7×4
28
3×4
12
28-12=16
So there will be 28 cars and 12 trucks.