I'm sorry, I understand shell theorem, but I still don't get why r/4 and 2r (which is also from the center of the planet) have the same gravitational acceleration. Can you please expound?
The given answer is g/4. But when I substituted R/4 into the radius, I get 16GM. Am I just using the wrong equation altogether? He also said that you also got g/4 if the distance was 2R.