3.47 Last year, 100 students at Century High School participated in the "AHSME", with an average score of 100. The number of non-senior participants was 50% more than the number of senior participants, and the average score of the senior participants was 50% higher than that of the non-senior participants. What was the average score of the senior participants?
(A) 100.
(B) 112.5.
(C) 120.
(D) 125.
(E) 150.
(42nd American High School Mathematics Examination, 1991)
[Solution] Let the number of graduating students participating in the competition be $x$, and the number of non-graduating students be $y$. According to the problem, we have
$$
\left\{\begin{array}{l}
x + y = 100, \\
y = \frac{3}{2} x.
\end{array} \text { Solving, we get } \quad \left\{\begin{array}{l}
x=40, \\
y=60.
\end{array}\right.\right.
$$

Furthermore, if the average score of the graduating students is $u$, and the average score of the non-graduating students is $v$, then
$$
\left\{\begin{array}{l}
40 u + 60 v = 100 \cdot 100, \\
u = \frac{3}{2} v.
\end{array} \text { Solving, we get } u=125.\right.
$$

Therefore, the answer is $(D)$.