(0.75×68.3-0.317×25+31.7）÷3=【0.75×68.3+31.7×（1-0.25）】÷3=【0.75×68.3+31.7×0.75】÷3=【0.75×（68.3+31.7）】÷3=0.75×100÷3=75÷3=25
