= Solution
Assume approximate <charge symmetry of nucleon parton distributions>, the interchange of <up quarks> and <down quarks> under the <isospin> rotation carrying the <proton> into the <neutron>:
$$
u^n=d^p,\quad d^n=u^p,\quad \bar u^n=\bar d^p,\quad \bar d^n=\bar u^p,\quad s^n=s^p,\quad\bar s^n=\bar s^p.
$$
Corrections from the <up quark>-<down quark> <mass> difference and the <electromagnetic interaction> are neglected. With the proton-only shorthand in <2/d>,
$$
\boxed{F_2^p=\frac{x}{9}(4U+D+S),\qquad F_2^n=\frac{x}{9}(4D+U+S).}
$$
If charm is active, add the same $4x(c^p+\bar c^p)/9$ to each; heavier flavors are treated similarly. These formulae use the same $Q^2$ and factorization convention for all <parton distribution functions>.
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