Acceleration due to gravity above Earth

"How far above the Earth's surface will the acceleration due to gravity be a quarter of what it is on the surface?"

I tried to use the g' = (G * m) / r^2 equation (with the mass of the earth as "m") because it was used with a somewhat similar problem in my book about the value of g on the top of Mount Everest, but I came up with a ridiculous number.

Can you tell me how to do this problem correctly?