分析:(1)運用“一個數(shù)減去兩個數(shù)的和,可以用這個數(shù)分別減去這兩個數(shù)”形式計算,使計算更加簡便.
(2),(3)運用乘法的分配律進行計算,使計算更加簡便.
(4)按照先計算括號內(nèi)部的再計算括號外面的運算順序進行計算.
(5)、(6)運用比例的基本性質(zhì)進行解答,即可求出未知數(shù)的值.
解答:解:(1)19.875-(5.5+8
),
=19.875-5.5-8.875,
=11-5.5,
=5.5;
(2)(
+)×23×19,
=
×19+
×23,
=1+1,
=2;
(3)27.2×
+3
×12.5%+0.125,
=
×(27.2+3.8+1),
=
×32,
=4;
(4)[
+(6.4-4
)×
]÷
,
=[
+(6.4-4.4)×
]×
,
=[
+5]×
,
=
×
,
=
,
=3
;
(5)12:15=0.4:X,
12X=15×0.4,
12X÷12=6÷12,
X=0.5;
(6)
=
,
2.5X=1.6×0.25,
2.5X÷2.5=0.16×2.5÷2.5,
X=0.16.
點評:本組題考查了四則混合運算順序和運算法則的運用,能靈活運用所學(xué)的運算定律簡便計算,同時還考查了比例的基本性質(zhì).