Tính nhanh : a) 6 x 7 x 8 x 9 x 10/8 x 10 x 12 x 3 x 4 =?. b) 2 x 3 x 4 x 5 x 10/8 x 8 x 18 x 10 x 12 x 3 x 4 =?. c) 7 x 3 x 8/9 x 14 x 5  =?. d) 3 x 4 x 25/5 x 21 x 3