Which has the greater capscity 7/8 of a cup. or 2/8 of a gallon

Well, there are 8oz in a cup, so in 7/8 of a cup there are 7/8 * 8 ounces or 7oz then.

now, a gallon has 128oz total, so  2/8 of a gallon will then be (2/8) * 128, or 32oz.

so, fairly sure you can tell who has greater capacity.

The density of the cube is 40