Draw three circles that interlock each other, and label them C (chorus), B (band), O (orchestra). This is the Venn diagram. The interlocking circles divide space into 7 regions:
a. Chorus only
b. Band only
c. Orchestra only
d. C and B only
e. B and O only
f. C and O only
g. All 3
Now put in figures. Combined letters mean addition of their region numbers.
adfg=33; bdeg=45; cefg=21; dg=15; fg=9; eg=4; g=2.
From these we get:
e=2, because g=2 and ge=4; f=7; d=13; c=21-2-7-2=10; b=45-13-2-2=28; a=33-13-7-2=11.
So (a,b,c,d,e,f,g)=(11,28,10,13,2,7,2). There are c=10 on the orchestra only.