17.25-（7.5-2.75）=17.25-7.5+2.75=17.25+2.75-7.5=20-7.5=12.5
简便计算17.25-(7.5-2.75)=17.25-7.5+2.75=17.25+2.75-7.5=20-7.5=12.5
