（78.6-0.786×25+75%×21.4）÷15×1997=[78.6-78.6×0.25+75%×（100-78.2）]÷15×1997；=（78.6-78.6×0.25+0.75×100-0.75×78.2）÷15×1997；=[（1-0.25-0.75）×78.6+0.75×100]÷15×1997；=75÷15×（2000...