Question

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Question
Which number is a Composite Number?





(1 point)
Responses

13
13

17
17

21
21

31
31
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explain but explain in a way I could write it or draw it on paper

Answer 1

Answer: 21

How to show it on paper (two simple ways):

1) Division/factors method (write these steps):
- A composite number has a factor other than 1 and itself.
- Test 21 by small primes: 21 ÷ 2 = not whole, 21 ÷ 3 = 7 (whole). So 21 = 3 × 7. Since it has factors 3 and 7 (besides 1 and 21) it is composite.
- For comparison, check 13, 17, 31 by dividing by 2, 3, 5 (and 7 if needed). None divide evenly, so those are prime.

2) Draw it as an array (draw on paper):
- Draw 3 rows of 7 dots:
●●●●●●●
●●●●●●●
●●●●●●●
That is a 3×7 rectangle (3 groups of 7), showing 21 can be arranged into more than a 1×21 line — therefore composite.
- If you try the same for 13, 17, 31 you cannot make a rectangular array with more than one row and more than one column (only 1×13, 1×17, 1×31), showing they are prime.