Notice the verb "precede" in the noun precedence, and you'll be able to remember that precedence is the act of preceding, of coming before — either in time or in rank.
When you're stranded without food or water, the search for water should definitely take precedence over the search for food, since you'll die of dehydration long before you starve. When deciding where to seat people, restaurant owners give precedence to the famous and rich: making them happy will be good for business. Which things take precedence over others depends on what you think is more important. "Age before beauty," I always say.
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