Well the Regular Price is the price before the sale was added. So here's the work that I would put in and in the end, just divide by (1 - 0.38).
Price x (1 - 0.38) = Sale Price (529)
Price x 0.62 = 529
Price = 529 / 0.62 = 853.23
$853.23 was the Regular Price
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LeBron James definitely lol
The variable is the group that was given caffine, because thats what the doctors manipulated. They possibly altered the natural memory of the participants by giving them caffine. A more specific variable could be how much caffine each participant is given and what time are they given it.