(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