25分之1311=52.44 20分之1591=79.55
25分之1311×79.45+159×47.56+20分之1591×52.44
=52.44×79.45+159×47.56+52.44×79.55
=52.44×（79.45+79.55）+159×47.56
=52.44×159+159×47.56
=159×（52.44+47.56）
=15900