分析:(1)根据加法交换律简算;
(2)先把28分解成4×7,再运用乘法结合律简算;
(3)根据乘法交换律简算;
(4)根据减法的性质简算;
(5)根据除法商不变的规律简算;
(6)根据除法的性质简算.
解答:解:(1)46+789+54,
=(46+54)+789,
=100+789,
=889;
(2)25×28,
=25×(4×7),
=25×4×7,
=100×7,
=700;
(3)15×13×4,
=15×4×13,
=60×13,
=780;
(4)421-173-27,
=421-(173+27),
=421-200,
=221;
(5)720÷45,
=(720÷9)÷(45÷9),
=80÷5,
=16;
(6)420÷35÷2,
=420÷(35×2),
=420÷70,
=6.
点评:本题考查了四则混合运算,注意运算顺序和运算法则,灵活运用所学的运算定律进行简便计算.