Express in roster form. Set of B is the set of all elements x such that x is an element of natural numbers and x is a multiple of 8

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Explanation:
The set of natural numbers is {1,2,3,...} basically anything positive and a whole number.
Any multiple of 8 is of the form 8x. Since x is a natural number, the smallest it can be is x = 1 which corresponds to 8x = 8*1 = 8. So 8 is the first multiple of the set. Then 16 is next because 8x = 8*2 = 16, and so on.
That's how we end up with {8, 16, 24, 32, ...}
The three dots, or ellipses, tell the reader that the pattern goes on forever. This set is infinitely large. We wouldn't stop at 800 because we could plug in say x = 200 to get 8x = 8*200 = 1600 and that's a multiple of 8.