There are three piles of chess pieces with the same number, each pile has at least 4 pieces, take 3 pieces from the left pile and put them in the middle pile, take 4 pieces from the right pile and put them in the middle pile, and then take out the same number of chess pieces from the middle pile as the left pile Put the number of chess pieces in the left pile, what is the number of chess pieces in the middle pile? Please do it and explain why.
有三堆棋子，数目相等，每堆至少有4枚，从左堆中取出3枚放入中堆，从右堆中取出4枚放入中堆，再从中堆中取出与左堆剩余棋子数相同的棋子数放入左堆，这时中堆的棋子数是多少？请做一做，并解释其中的道理．
<gadget id="calculator">7 + 3</gadget>
<output>10</output>

<result>10</result>