80x＋100(20－x)＝100x+80(20－x)＋40
⬇開く
80x＋2000－100x＝100x＋1600－80x＋40
⬇移項する
80x－100x－100x＋80x＝1600＋40－2000
⬇左辺と右辺をそれぞれ計算
－120x＝360
⬇xを解く（両辺に－1/120をかける）
－120x（×－1/120）＝360(×－1/120)
⬇よって
x＝－3
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