Question 1201393
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Venn Diagram
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circle(-1,1,2),circle(1,1,2),circle(0,-0.7321,2),

locate(-3.2,2.8,Econ),
locate(2.6,2.8,Geo),
locate(1,-2.6,Math),
locate(-2,1.8,a),
locate(0,1.8,b),
locate(2,1.8,c),
locate(-1.5,0,d),
locate(0,0.4,e),
locate(1.2,0,f),
locate(0,-1.5,g),
locate(3,-1.5,h)

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There are 8 distinct regions
a = Econ only
b = Econ and Geo, but not Math
c = Geo only
d = Econ and Math, but not Geo
e = all 3
f = Geo and Math, but not Econ
g = Math only
h = neither of the 3 classes mentioned


Given facts
<table border = "1" cellpadding = "5"><tr><td>Number</td><td>Fact</td></tr><tr><td>1</td><td>70 students</td></tr><tr><td>2</td><td>6 offer Economics only</td></tr><tr><td>3</td><td>18 offer Geography and economics but not Mathematics</td></tr><tr><td>4</td><td>36 offer Economics and Mathematics</td></tr><tr><td>5</td><td>53 offer Economics</td></tr><tr><td>6</td><td>50 offer Geography</td></tr><tr><td>7</td><td>34 offer Mathematics.</td></tr><tr><td>8</td><td>All students offer at least one subject.</td></tr></table>


Fact 2 leads directly to a = 6
Fact 3 tells us b = 18
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locate(-3.2,2.8,Econ),
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locate(-2,1.8,6),
locate(0,1.8,18),
locate(2,1.8,c),
locate(-1.5,0,d),
locate(0,0.4,e),
locate(1.2,0,f),
locate(0,-1.5,g),
locate(3,-1.5,h)
)

}}}
fact4 says that d+e = 36


we have
a = 6
b = 18
d+e = 36
so,
a+b+d+e = 6+18+36 = 60


But this sum is larger than the number of people taking Econ (see fact 5). We run into a contradiction.


Put another way:
fact5 says that a+b+d+e = 53
Let's plug a = 6 and b = 18 into that and isolate d+e
a+b+d+e = 53
6+18+d+e = 53
24+d+e = 53
d+e = 53-24
d+e = 29
This clashes with fact 4 that d+e = 36


Either way we run into a contradiction. 
It's likely your teacher made a typo somewhere when coming up with the numbers to this problem. 
I would ask him/her for further clarification.
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