Equation for Radii

Hi,

Have a question about equations, I'm constructing a sketch with two radii where I want one to be 2/3 the size of the other one but not assigning a dimension to the bigger one. Is it possible to put an equation to drive the radii to work out so that there is that difference between them. I've attached an image so you can see the type of sketch I want to create.

James

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