Question 1140169
This is a Venn diagram with 2 intersecting
circles. There is 1 person outside of both circles
( math and science circles ), so {{{ 5 - 1 = 4 }}}
are in the circles. Label as follows:
X = only studies math
Y = studies both 
Z = only studies science
(1) X + Y + Z = 4
(2) X + Y = 3 ( given )
(3) Y + Z = 4 ( given )
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From (1) and (3), X must be zero
So, from(2), Y = 3 , and plug this into (3)
Z = 1
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So, 3 people are studying both.
Get another opinion if needed