= Solution
Let $k_D$ be the donor's total decay rate without acceptors, including radiative and nonradiative decay, and let $k_T$ be its transfer rate to one acceptor. In weak excitation, independent equivalent acceptors supply additive transfer rates, so <FRET efficiency> is
$$
E=\frac{k_T}{k_D+k_T},\qquad
E_n=\frac{nk_T}{k_D+nk_T}.
$$
Eliminating $k_T/k_D=E/(1-E)$ gives
$$
\boxed{E_n=\frac{nE}{1+(n-1)E}}.
$$
This assumes the same <Förster distance>, equivalent donor–acceptor distances and unchanged donor decay, with no acceptor saturation or other transfer pathways affecting donor de-excitation.
For <Förster resonance energy transfer>, $k_T/k_D=(R_0/R)^6$, hence
$$
E_n=\frac{n(R_0/R)^6}{1+n(R_0/R)^6},\qquad
\boxed{R=R_0\left[\frac{n(1-E_n)}{E_n}\right]^{1/6}
=6(2/3)^{1/6}\,\mathrm{nm}\simeq5.61\,\mathrm{nm}}.
$$
Here the single-pair <FRET efficiency> would be $E=0.60$, which indeed produces $E_2=0.75$.
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