Question 1172111
p(both)=p(c)*p(s) if independent
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p(c or s)=p(c)+p(s)+p(both), 
so putting it in student number
p(c or s)=90+70-55 (55 is double counted)
and 160-55=115 students that like one or the other.
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so 55/140=(90/140)* (70/140) if independent events
55/140=45/140, which it isn't, so not independent events