（0.8+四分之一）/七分之三+0.7
=(0.8+0.25)/(3/7)+0.7
=1.05*7/3+0.7
=0.35*7+0.7
=2.45+0.7
=3.15
（0.8+四分之一）/七分之三+0.7
(0.8+1/4)/3/7+0.7
=1.05/3/7+0.7
=1.05*7/3+0.7
=7.35/3+0.7
=2.45+0.7
=3.15
（楼上题都看错了）
先换算：
（0.8+0.25）/3/7+0.7
=1.05/3/7+0.7
=0.35/7+0.7
=0.05+0.7
=0.75
0.8=4/5 0.7=7/10
(4/5+1/4)/3/7+0.7
=21/20*7/3+0.7
=49/20+14/20
=63/20
=3.15
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