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Math·Algebra·Linear equations in one variable

hard

A machine makes 4-inch, 7-inch, and 9-inch parts. During a certain day, the number of 4-inch parts that the machine makes is 5 times the number of 7-inch parts, and the number of 9-inch parts is 4. During this day, the machine makes 100 parts total. Which equation represents this situation, where $n$ is the number of 7-inch parts?

A

$4(5n)+7n+9(4)=100$

B

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

C

$5n+4=100$

D

$6n+4=100$