Question 780916
Draw 2 circles that intersect. 
One circle has 21 in it that wear t-shirts
The other circle has 32 in it that wear jeans
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The intersection has 15 in it that wear both
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{{{ 21 - 15 = 6 }}} is the number that wear 
t-shirts but not jeans
{{{ 32 - 15 = 17 }}} is the number that wear
jeans but not t-shirts
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Notice that {{{ 17 + 15 + 6 = 38 }}} which is all the students