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Working alone at its constant rate, machine A produces k car parts in 10 minutes. Working alone at its constant rate, machine B produces k car parts in 15 minutes. How many minutes does it take machines A and B, working simultaneously at their respective constant rates, to produce k car parts? ____________minutes
这类GRE OG题目小学时候经常遇到，相信很多人都是这种感受哈
1/(1/10+1/15)=6min