The total number of marbles in the bag is 3 + 2 + 5 = 10. The number of marbles that are either red or blue is 3 + 2 = 5. So the fraction of marbles that are either red or blue is 5/10, which can be simplified to 1/2. To convert 1/2 to a decimal, I can divide the numerator by the denominator. 1 divided by 2 is 0.5. So the probability of drawing one marble which is either red or blue from the bag is 0.5. 
What is the probability, expressed as a decimal, of drawing one marble which is either red or blue from a bag containing 3 red, 2 blue, and 5 yellow marbles?