There are 16 marbles total: (6 red, 6 white, 4 blue)
Since we are interested in the marble being either red or blue, we know that 10 out of 16 marbles will satisfy our conditions
That being said:
The probability of selecting a red marble is 6/16
The probability of selecting a blue marble is 4/16
Since we don't care whether the marble is red or blue, as long as it is either one, we simply have to add the probability of selecting red and the probability of selecting blue:

This simplifies to:
