Systems of equations over sets of natural numbers (or, equivalently, language equations over a one-letter alphabet) of the form X_i = φ_i(X₁,...,X_n) (1 ≤ i ≤ n) are considered. Expressions φ_i may contain the operations of union, intersection and pairwise sum A+B = {x+y | x ∈ A, y ∈ B}. A system with an EXPTIME-complete least solution is constructed, and it is established that least solutions of all such systems are in EXPTIME. The general membership problem for these equations is proved to be EXPTIME-complete.