Machine $A,$ working alone at a constant rate, can complete a certain production lot in $x$ hours. Machine $B,$

Machine $A,$ working alone at a constant rate, can complete a certain production lot in $x$ hours. Machine $B,$ working alone at a constant rate, can complete $\dfrac15$ of the same production lot in $y$ hours. Machines $A$ and $B,$ working together, can complete $\dfrac12$ of the same production lot in $z$ hours. What is the value of $y$ in terms of $x$ and $z?$

A. $\dfrac{5x - 10z}{2xz}$
B $\dfrac{2xz}{5x - 10z}$
C. $\dfrac{5xz}{x + z}$
D. $\dfrac{xz}{x} + z$
E. $\dfrac{xz}{x} + 2z$

Answer: B

Source: Manhattan GMAT

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