Walter and Weckesser's result (Aequationes Math 46:212-219, 1993), extending the Bushell-Okrasiński convolution type inequality (Bushell and Okrasiński in J Lond Math Soc (2) 41:503-510, 1990), gave some general conditions on the functions k : [0, d) → R and g : [0, ∞) → R under which, for every increasing function f : [0, d) → [0, ∞), the inequality x 0 k (x − s) g (f (s)) ds ≤ g x 0 f (s) ds , x ∈ (0, d) , is satisfied. Applying the result on a simultaneous system of functional inequalities, we prove that if d > 1, then, in general, both k and g must be power functions.