A box contains 18 marbles, 8 red, 4 green, & 6 white. What is the smallest number of marbles that must be selected to have one of each color marble?
To find the smallest number of marbles that must be selected to have one of each color marble, we can take an approach by considering the worst-case scenario.
First, let's assume we select all the marbles except for one color. In this case, let's assume we don't select any white marbles. So, we'd have selected all the 8 red marbles and 4 green marbles, which totals to 12 marbles.
Now, if we select one more marble, that marble can only be white since all the other colors are already selected. Therefore, the smallest number of marbles that must be selected to have one of each color marble is 12 + 1 = 13 marbles.
So, the answer is 13 marbles.