We have a small deck of ten cards. Five of the cards are red, three are blue and two are green. We randomly select four cards from the deck with replacement. That is, after we select a card and record the colour, we put the card back in the deck and thoroughly shuffle them before we select another card.
There are three possible events to consider:
1. P(red + red) = 5/10 * 5/10
2. P(blue + blue) = 3/10 * 3/10
3. P(green + green) = 2/10 * 2/10
These three events are mutually exclusive, therefore the probability of the first two selected cards being the same color is the sum of the above three probabilities.