Question

in a bag of marbles, you have 3 green, two yellow, six blue, and nine red marbles. What is the probability of picking a red marble from the bag?

Answer 1

So first add all of the marbles together: 3+2+6+9=20
Because there are 20 marbles together, that would be your denominator. 
All of the other numbers would be your numerator. So you would have:

3/20 green marbles
2/20 yellow marbles
6/20 blue marbles
9/20 red marbles

So your answer would be the probability of picking a red marble from the bag is 9/20 or 45%