At a certain depth " \(d\) " below surface of earth. value of acceleration due to gravity becomes four times that of its…

At a certain depth " \(d\) " below surface of earth. value of acceleration due to gravity becomes four times that of its value at a height \(3 R\) above earth surface. Where \(R\) is Radius of earth (Take \(R=6400 km\) ). The depth \(d\) is equal to;