Tính nhanh, 4/7 x 7/9 - 4/7 x 2/9 - 4/7 x 5/9; 15/17 : 7/9 + 7/17 : 7/9 - 5/17 : 7/9

tính nhanh:  b) 4/7 x 7/9 - 4/7 x 2/9 - 4/7 x 5/9

c) 15/17 : 7/9 + 7/17 : 7/9 - 5/17 : 7/9