Calculate the average number of images with an attribute class of "keyboard".
AVG(IMG_ID) where OBJ_CLASS = 'keyboard';
SELECT AVG(T1.IMG_ID) FROM IMG_OBJ AS T1 INNER JOIN OBJ_CLASSES AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID WHERE T2.OBJ_CLASS = 'keyboard'