The gravitational force between two masses, M1 and M2, at distance r apart, is \(\displaystyle \frac{GM1M2}{r^3}\). Imagine a 'test mass" of mass 1 kg and let "M" be the mass of the earth, "R" the radius of the earth. Such a test mass at distance r from the center of the earth would feel a gravitational force \(\displaystyle \frac{GM}{r^2}\) from the earth. Assuming that mass is on the line between the centers of the moon and the earth, its distance from the moon would be 60R- r and the mass of the moon is M/81. So the gravitational force on the object from the moon would be \(\displaystyle \frac{\left(\frac{GM}{81} \right ) }{(60R- r)^2}= \frac{GM}{81(60R- r)^2}\). Set those equal and solve for r. (It will be in terms of R, of course. Both G and M will cancel.)