A group of boys has an average weight of $36 \,kg$. One boy weighing $42 \,kg$ leaves the group and another boy weighing $30\, kg$ joints the group. If the average now becomes $35.7\, kg$, then how many boys are there in the group?

  • A

    $30$

  • B

    $32$

  • C

    $40$

  • D

    $56$

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