1本詞典換回2本字典還要付7.3元字典
那麽 1本詞典+7.3元=2本字典
3本詞典就價格+7.3X3=6本字典
(147.5+7.3x3)/(5+3x2)
=169.4/11
=15.4元
字典15.4元,
詞典2x15.4-7.3=23.5