a.9/20-8/15x5/12
=9/20-2/9
=41/180
b.2/3:4/5:7/12
=5/6:7/12
=10/7
c.4/9+11/8-5/6
=131/72-5/6
=71/72
tk mk nhé bạn
9/20 - 8/15 x 5/12 = 9/20 - 2/9
= 81/180 - 40/180
= 41/180
2/3 : 4/5 : 7/12 = 2/3 x 5/4 : 7/12
= 5/6 : 7/12
= 5/6 x 12/7
= 10/7
4/9 + 11/8 - 5/6 = 32/72 + 99/72 - 60/72
= 71/72