800÷400= 40÷20= 160÷80= 8000÷2000= 4000÷2000= 2400÷1200= ．
(1–×)÷ [(1–)×]÷4
[(1–)×]÷4++
(–)÷(+)[1–(+)]÷3.5
25÷(–)+–
×
)÷
[(1–
)×
)×
]÷4
+
+
–
)÷(
+
)[1–(
+
)]÷3.5
–
)
+
–
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