Rachel bought a collection of buttons.
2/4 of the buttons are yellow. 2/4 * 8 = 1/2 * 8 = 4 yellow buttons
2/8 of the buttons are red. 2/8 * 8 = 1/4 * 8 = 2 red buttons
Therefore, because we know that 4 of the buttons are yellow and 2 of the buttons are red, and we know that 4 > 2, we know that more yellow buttons were bought.