Geared Mechanism design

Hello Everybody!
I want some help designing a mechanism, I hope someone here could do that. The description is given below.
I have got two concentric shafts. Outer shaft is output shaft & inner is input shaft.
Both shafts rotate in the same direction.
Outer shaft rotates with half the speed of inner shaft.
Dia of outer shaft is 80mm & inner shaft is of 50mm.
Only one pair of gears is to be used.
How I am trying to solve this problem is that one internal spur gear should be attached to outer shaft and one spur gear with inner shaft but at some off centre(i.e. at some distance from the centre of the shaft).
Suggestions required.

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