Donovan reaches into a jar containing 6 red marbles, 3 blue marbles, 4 yellow marbles, and 5 green marbles,
What is the probability that a red marble is not chosen?

12 out of 18. This is because there is 6 red marbles and 18 of them all together. The question states the probability of the red marble NOT being chosen therefore you need to take away 6 from 18 leaving you with 12. I hope this made sense :)