Ambrose has a bag of marbles with R red marbles, G green marbles, B blue marbles, and Y yellow marbles. Write an expression for each of the following situations.

1. The probability of selecting a yellow marble.

   Answer (a):

   $\frac{Y}{R+G+B+Y}$

2. The probability of selecting a blue or green marble.

   Hint (b):

   Add the blue and green together.

3. The probability of selecting a purple marble.

   Hint (c):

   How many purple marbles are there?