Two pipes $A$ and $B$ can separately empty a cistern in $12$ $hours$ and $15$ $hours$,respectively. In what time will the cistern be emptied,if both the pipes are opened together?

  • A
    $5 \text{ hours } 30 \text{ minutes}$
  • B
    $7 \text{ hours}$
  • C
    $6 \text{ hours } 40 \text{ minutes}$
  • D
    $\text{None of these}$

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