According to all the observations of Elly Koss, what was her average weight?
DIVIDE(SUM(VALUE), COUNT(VALUE)) WHERE DESCRIPTION = 'Body Weight';
SELECT AVG(T2.VALUE), T2.units FROM patients AS T1 INNER JOIN observations AS T2 ON T1.patient = T2.PATIENT WHERE T1.first = 'Elly' AND T1.last = 'Koss' AND T2.description = 'Body Weight'