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Math·Algebra·Linear equations in one variable

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A machine makes $8$-inch, $5$-inch, and $7$-inch parts. During a certain day, the number of $8$-inch parts that the machine makes is $3$ times the number $n$ of $5$-inch parts, and the number of $7$-inch parts is $12$. During this day, the machine makes $100$ parts total. Which equation represents this situation?

A

$8(3n)+5n+7(12)=100$

B

$8n+5n+7n=100$

C

$3n+12=100$

D

$4n+12=100$