Enter the following custom calculation script in the Sum field and change the field names to match your check boxes:
event.value<20?this.getField("Check Box1").checkThisBox(0,true):this.getField("Check Box1").checkThisBox(0,false);
(event.value>19 && event.value<25)?this.getField("Check Box2").checkThisBox(0,true):this.getField("Check Box2").checkThisBox(0,false);
(event.value>24 && event.value<31)?this.getField("Check Box3").checkThisBox(0,true):this.getField("Check Box3").checkThisBox(0,false);
(event.value>40 && event.value<46)?this.getField("Check Box4").checkThisBox(0,true):this.getField("Check Box4").checkThisBox(0,false);
event.value>45?this.getField("Check Box5").checkThisBox(0,true):this.getField("Check Box5").checkThisBox(0,false);