Welcome to Bytelearn!

Let’s check out your problem:

Machine $X$, working alone at its constant rate, produces $600$ parts in $3$ hours. Machine $Y$, working alone at its constant rate, produces $600$ same parts in $2$ hours. Machine $Z$, working alone at its constant rate, produces $600$ same parts in $6$ hours. To produce $600$ same parts, Machine $X$ and $Y$ worked together for $600$$2$ hour, and then Machine $Z$ worked alone. For how many hours would Machine $Z$ run to produce the remaining parts?