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.  

Therefore d=8, so we replace d by 8

Therefore a+b=38

Therefore b+c=28

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

 

That's the 4 in the middle region.

4 out of 70 is the fraction .

Edwin