If a bag contains 2 red marbles, 3 blue marbles, 5 yellow marbles and 2 green marbles, if a marble is randomly selected what is the probability that it is red

All you need to do here is just find the ratio of red marbles to all of the marbles.
                    2 red                       
2 red + 3 blue + 5 yellow + 2 green

when you add everything up, you get 2/12, which is equal to 1/6.

So red, blue, yellow, green

2 red
3 blue
5 yellow
2 green

probability=

the desired outcomes is red or 2

total possible is all marbles or 2+3+5+2=12

so the probability of picking a red marble is 2/12=1/6