First, set up the ratio of c:t, which is 12:8. Then set up your new ratio of c:t, which is 105:?. The way I find the easiest is to divide 105 by 12, which is 8.75, then multiply that by 8, which is 70. To check if it's right, just divide 12 by 8, and 105 by 70, and if they're the same number, you've got it right.
Ps. I'm guessing you meant to ask the minimum number of trucks, not cars.
