Question

at what height from the ground??

Answer 1

Same changes in value of ′g′ can be observed at a depth ′x′ and height ′2x′

given that the depth ′d′=x=10km

∴ The same change can be observed at a height 2x=20km

Hence a is the correct answer

Answer 2

I think 20 km . Hope it will help you.