You can use:

Then for each step you multiply the previous step by R. For example; if A = 3 and B = 7, and you want to get from A to B in four steps (Z=4) this equation yields:

and the steps are;

3,

3 x 1.236 = 3.708,

3.708 x 1.236 = 4.582,

4.582 x 1.236 = 5.664,

5.664 x 1.236 = 7