I'm about as far from a mathematician as you can get, but, wouldn't this work?
Motor w/3" pulley @ 5000 RPM -> 7" pulley = 43% reduction or 2150 shaft speed driving 10" sprocket (@ 2150 rpm) ->23" sprocket = another 43% reduction or 925 RPM @ drive shaft.
I'm sure there is an algebra formula that would get ya there a lot quicker, but unless I'm missing something........
Regards,
hm
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