Answer: He can completely decorate 11 cars
Explanation: since he puts 2 snowflakes on each of the 4 windows on one car, in total Jack uses 8 snowflakes per car. If he makes 90 snowflakes, 90 divided by 8 gives you the amount of cars he can decorate. 90/8 = 11.25, but since you can’t have .25 cars the answer is 11.