$30 \cdot 5$ a two-digit number, using the sum of the units and tens digits of this number multiplied by 3 and then subtracting -2, still results in the original number. This two-digit number is
(A) 26.
(B) 28.
(C) 36.
(D) 38.
(2nd "Hope Cup" National Mathematics Invitational Competition, 1991)
[Solution] Let the two-digit number be $\overline{a b}$. According to the problem, we have
$$
3(a+b)+2=10 a+b \text {, i.e., } 7 a=2 b+2=2(b+1)
$$

It is clear from the above equation that $a$ is even, and $b+1$ is a multiple of 7.
Therefore, the answer is $(A)$.