2019年第23届广东世界少年奥林匹克数学竞赛三年级竞赛初赛第10题5分
已知$$\triangle +\triangle =\bigcirc +\bigcirc +\bigcirc $$，$$\bigcirc +\bigcirc +\bigcirc =\square +\square +\square $$，$$\bigcirc +\square +\triangle +\triangle =100$$，则$$\triangle =$$． 
拓展思维->拓展思维->计算模块->方程基础->等量代换->不可直接计算的算式代换
因为$$\triangle +\triangle =\bigcirc +\bigcirc +\bigcirc $$即$$2\triangle =3\bigcirc $$；  $$\bigcirc +\bigcirc +\bigcirc =\square +\square +\square $$即$$3\bigcirc =3\square $$，则$$\bigcirc =\square $$；  $$\bigcirc +\square +\triangle +\triangle =\bigcirc +\bigcirc +3\bigcirc =5\bigcirc =100$$，  所以$$\bigcirc =20$$，则$$\square =20$$，$$\triangle =30$$．  故选：$$\text{C}$$． 