Answers

2016-07-03T14:06:31+08:00
12 used twitter and 3 used both..
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2016-07-03T14:10:21+08:00
Using the data given you'll have:
Total number of students = 25
Number of Students using FB = 15
Number of Students using T = 12
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To get the number of students that are using both of the 2 apps you'll have 
n = (15 + 12) - 25
 n = 27 - 25
n = 2
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To get the number of students who are using Twitter only you'll have:
t = 12 - n
t = 12 - 2
t = 10
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