calculus

There are different approaches for these problems, but one way to look at it is to think about how many boxes can each belt do in 1 minute.

Conveyor A moves 10009\displaystyle \frac{1000}{9} boxes in 1 minute

Conveyor B moves 10007\displaystyle \frac{1000}{7} boxes in 1 minute

Conveyor C moves 1000x\displaystyle \frac{1000}{x} in 1 minute.

All together they move 10003\displaystyle \frac{1000}{3} in 1 minute.

Put it all together and solve for x.
 
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