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Math·Algebra·Linear equations in one variable

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A machine makes $9$-inch, $4$-inch, and $7$-inch parts. During a certain day, the number of $9$-inch parts that the machine makes is $3$ times the number $n$ of $4$-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

$9n+4n+7n=100$

B

$4n+12=100$

C

$9(3n)+4n+7(12)=100$

D

$3n+12=100$