Using Empirical Rule (68 - 95 - 99.7 Rule), 68% of the data lies in between 1 standard deivation from the mean so since you are not doing 55 to 65 and instead 60 to 65, you divide 68 by 2 and you get 34%.
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The volume of one box is 240 inches cubed, because 12x8x2.5=240. If we multiply this by 2, then the volume of two boxes of detergent is 480 inches cubed.