First we can realize that the number of cubes of each type does not matter, only the ratios between them. We see that if there are x yellow cubes, there are 3x blue cubes. And therefore, there are 5*3x green cubes, or 15x green cubes. This means that the cubes are in a 1:3:15 ratio, or that for every 1 yellow cube, there are 3 blue and 15 green cubes.
Now we can find the probability of picking a yellow cube. Since there are 18 non-yellow cubes for every yellow cube, this means that there is a 1/19 possibility of picking a yellow cube.
