Answer:
D. 7(1+3)
Explanation:
7+21
You split it into two factors using the distributive property. The common factor in both is 7.
7 goes into itself once, and goes into 21 three times. Thats where the 1 and 3 come from.
28
7 + 21 can be rewritten as 7(1 + 3), or 7(4), or 28
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