I think your teacher wants you to use a Venn diagram (also known as Euler
diagram) like this:
The red circle represents a set R that contains all those who own a radio.
The blue circle represents a set T that contains all those who own a TV.
The left region of the red circle that has "a" in it are those who own
a Radio but do not own a TV.
The middle region that has a "b" is part of both circles, and represents
a set that contains all those who own BOTH a radio and a TV.
The right region of the blue circle that has "c" in it are those who own
a TV but do not own a radio.
The region that has "d" in it is outside of both circles. It represents
the ones which have neither a radio nor a TV.
If 8 students don't own either of them,
Therefore d=8, so we replace d by 8
38 own a radio
Therefore a+b=38
and 28 own a TV.
Therefore b+c=28
out of 70 students,
Therefore a+b+c+d=70, and since d=8,
a+b+c+8=70 or a+b+c=62
Subtracting these equations:
a+b+c=62
a+b =38
--------
c=24
Substituting in b+c=28
b+24=28
b=4
Substituting in
a+b=38
a+4=38
a=34
So the complete Venn diagram is
what fraction of the students owns both a radio and TV
That's the 4 in the middle region.
4 out of 70 is the fraction 
.
Edwin
. 