18:5
分析:由题意可知,大圆面积的(1-

)等于空白面积,小圆面积的(1-

)等于空白面积,即大圆面积×(1-

)=小圆面积×(1-

),根据比例的基本性质求出大圆和小圆面积的比,进而求出大圆阴影面积与小圆阴影面积的比.
解答:由分析可知:大圆面积×(1-

)=小圆面积×(1-

),
则:大圆面积:小圆面积=

:

=20:7,
则大圆阴影部分面积与小圆阴影部分面积之比是(20×

):(7×

)=18:5.
故答案为:18:5.
点评:解答此题的关键是:找出大圆面积的(1-

)等于小圆面积×(1-

),根据比例的基本性质分析、解答即可.