A long solenoid with $1000 \mathrm{turns} / \mathrm{m}$ has a core material with relative permeability 500 and volume $10^3 \mathrm{~cm}^3$. If the core material is replaced by another material having relative permeability of 750 with same volume maintaining same current of 0.75 A in the solenoid, the fractional change in the magnetic moment of the core would be approximately $\left(\frac{x}{599}\right)$. Find the value of $x$.