小学数学毕业考试问题

解:设进价为X元还价Y元.............................................................................................................

由题意得:刚好盈利时400-Y=(1+20%)X.........................................................................................

化简得:Y=-1.2X+400....................................................................................................................

又∵(1+50%)X≤400≤(1+100%)X..................................................................................................

∴解得:200≤X≤800／3.................................................................................................................

∵Y=-X+400................................................................................................................................

∴Y随着X的增大而减小................................................................................................................

∴当X=200时,Y最大=-1.2×200+400=160.....................................................................................

当X=800／3时,Y最小=-1.2×800／3+400=80.............................................................................

答;最多还价160元,最少还价80元..................................................................................................