解:(1)2:4:8,
=(2÷2):(4÷2):(8÷2),
=1:2:4;
(2)18:27:45,
=(18÷6):(27÷9):(45÷9),
=2:3:5;
(3)

,
=(

):(

):(

×6),
=3:2:1;
(4)

,
=(

×8):(

×8):(1×8);
=9:5:8;
(5)0.3:0.15:0.45,
=(0.3×

):(0.15×

):(0.45×

),
=2:1:3;
(6)1.6:2.4:4,
=(1.6×

):(2.4×

):(4×

),
=2:3:5.
故答案為:1:2:4,2:3:5,3:2:1,9:5:8,2:1:3,2:3:5.
分析:根據比的基本性質作答,即比的前項和后項同時乘或除以一個數(0除外)比值不變.
點評:此題主要考查了化簡比的方法,另外還要注意化簡比的結果是一個比,它的前項和后項都是整數,并且是互質數.