The general condition for the Fermi energy is,
$$n=\int\limits_{-\infty}^{E_F}D(E)dE.$$Here $n$ is the electron density and $D(E)$ is the density of states. For free electrons in a magnetic field at very low temperatures, the number of completely filled Landau levels will be the integer part of $\frac{n}{D_0}$. The highest occupied Landau level is partially filled and is at the Fermi energy.
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