There was a similar question posted few days back ...
The answer is 3 .
x study both A &B
y study both A &C
z study both B&C
so the people ONLY studyA is 10 -( 3 +x+y)
the people ONLY study B is 11 -(3 +x +z)
the people ONLYstudy C is 14 -(3 +y +z)
But according to question only 20 people study only one subject so
if we add all the 3 above we get 26 -2 *(x+y+z) = 20
it means x + y+ z = 3
so 3 people study 2 kinds .
this OE is the clearest one. Thus OA is 3
I just want to complement a little to the bold part:
x= study both A and B without C
y= study both A and C without B
z= study both B and C without A