  Let $1<c<d$ be two relatively prime integers and $g_{c,d}=cd-c-d$. We
confirm, by employing the Hardy--Littlewood method, a 2020 conjecture of
Ram\'{\i}rez Alfons\'{\i}n and Ska{\l}ba which states that $$#\left\{p\le
g_{c,d}:p\in \mathcal{P}, ~p=cx+dy,~x,y\in \mathbb{Z}_{\geqslant0}\right\}\sim
\frac{1}{2}\pi\left(g_{c,d}\right) \quad (\text{as}~c\rightarrow\infty),$$
where $\mathcal{P}$ is the set of primes, $\mathbb{Z}_{\geqslant0}$ is the set
of nonnegative integers and $\pi(t)$ denotes the number of primes not exceeding
$t$.
