The red circle contains the 60 photographers that take black and white
photographs.  They are split between the regions a and b, and we don't yet
know how many of the 60 go in region a and how many go in region b.

The blue circle contains the 75 photographers that take color photographers.
They are split between the regions b and c, and we don't yet know how many
of the 75 go in region b and how many go in region c.

The three regions a,b and c contain:
1. The photographers in region a take ONLY black and white photographs. 
2. The photographers in region b take BOTH black and white photographs and
   color photographs.
3. The photographers in region c take ONLY color photographs.

Now we have something definite. We know that region b contains 45
photographers.  So we write 45 in region b, the middle region, the
overlapping part of the two circles:



Now since we know that the red circle contains 60 photographers and that
the overlapping part contains 45, we know that region a contains the other
60-45 or 15 photographers.  So we put 15 in the leftmost region, region a:



Now we see that the red circle contains a total of 60 photographers. Since
we know that the blue circle contains 75 photographers and that the
overlapping part b contains 45, we know that region a contains the other 
75-45 or 30 photographers.  So we put 30 in the rightmost region, region c:



Now you see that the blue circle contains a total of 75 photographers.  The
question is:

Answer: 30

Edwin