$15 \cdot 1$ A school's wind band consists of 100 boys and 80 girls, and the string band consists of 80 boys and 100 girls. In total, 230 students participated in these two bands, with 60 boys participating in both. Therefore, the number of girls who only participated in the wind band and not in the string band is 
(A) 10.
(B) 20.
(C) 30.
(D) 50.
(E) 70.
(7th American Junior High School Mathematics Examination, 1991)
［Solution］Let the number of people who participate in both bands be $x$, as shown in the shaded area in the figure. According to the problem,

we have
$180+180-x=230$, then $x=130$.
$\because$ The number of boys participating in both bands is 60, then the number of girls participating in both bands is 70.
$\therefore$ The number of girls who only participate in the orchestra is $80-70=10$. Therefore, the answer is $(A)$.