There are two factories producing the same kind of car parts. There are $36$ workers in factory $A$, and every worker produces $81$ parts on average. Each worker produces $101$ parts on average in factory $B$. Each worker can produce $89$ parts on average in two factories together. How many workers are there in factory $B$? 
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$$36\times (89-81)\div (101-89)=24$$ 