设杯子原来共有M克糖水,则有0.2M克糖和0.8M克水.
倒走20%水,即损失了0.04M克糖和0.16M克水；
还剩下0.16M克糖和0.64M克水,又兑满水即加入0.2M克水；
此时杯中,0.16M克糖和0.84M克水.
现在的浓度是w=0.16M/M*100%=16%.