(1)216+35+84=35+(216+84),
(2)298-35-165=298-(35+165),
(3)400÷25÷4=400÷(25×4),
(4)a×6+6×15=6×(a+15),
故答案依次为:216,84,35,165,25,4,a,15.
(1)216+35+84=35+(216+84),
(2)298-35-165=298-(35+165),
(3)400÷25÷4=400÷(25×4),
(4)a×6+6×15=6×(a+15),
故答案依次为:216,84,35,165,25,4,a,15.