Question 184848
What you have to draw is 3 interlocking circles with
an area in the middle which is common to all 3.
Kind of like the olympic rings.
1 ring will be consumers who bought refrigerators
1 ring will be consumers who bought cars
1 ring will be consumers who bought washers
All the small and big areas making up the rings adds up to
the total number of people buying items 
Right away I'm told that the car ring contains 12 people,
the fridge ring contains 18 people, and the washer ring
contains 24 people
What I do is label each ring just outside of it like this:
{{{c=12}}}
{{{f=18}}}
{{{w=24}}}
Of these, 6 were going to buy both a car and a refrigerator
Locate the small area where c and f overlap, and put a {{{6}}}
inside it.
4 were going to buy a car and a washer.
This is the area common to c and w. Put a 4 inside it
10 were going to buy a washer and a refrigerator
Label the intersection of w and f with a 10
One person indicated that she was going to buy all three items. 
This last fact gives you all the answers. It's the key.
The intersection of c and f = 6, but 1 of those is the last
person, so 6 - 1 = 5 persons will buy ONLY both f and c.
10 - 1 = 9 persons will buy ONLY both w and f.
4 - 1 = 3 persons will buy ONLY both w and c.
Now you should be able to label every area on the diagram
(a) How many were going to buy only a car? 3
(b) How many were going to buy only a washer? 11
(c) How many were going to buy only a refrigerator 3
(d) How many were going to buy a car and a washer but not a refrigerator? 3
(e) How many were going to buy none of these items? 40
I come up with a total of 35 persons buying items
c = 3
w = 11
f = 3
w+c only = 3
f+c only = 5
w+f only = 9
All 3 = 1
------------
35 total
The trick is not to count the sme area twice.
Hope I got the right answers and my method helps you