cap s is a subset within a universal set, cap u If cap s is equal to x comma y comma 4 comma 9 comma ? , which could describe cap u ?

A universal set must include every element in the subset. The elements x, y, 4, 9, and ? can all be found as keys on a keyboard. They are not all numbers, letters, or punctuation marks, so “keys on a keyboard” could describe the universal set.

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