Calculate probability of either of two events occurring based on frequency v2

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Question 1

Out of $\var{ptotal}$ people, $\var{p1}$ use Twitter, $\var{p2}$ use Facebook, $\var{p3}$ use both, and $\var{p4}$ use neither.

Calculate the probability that a person selected at random uses Facebook. Write your answer as a fraction.

$\displaystyle{}$ Expected answer:

$\displaystyle{\frac{ 43 }{ 91 }}$

Score: 0/1.5

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Calculate the probability that a randomly selected person uses Facebook or Twitter. Give your answer as a fraction.

$\displaystyle{}$ Expected answer:

$\displaystyle{\frac{ 11 }{ 13 }}$

Score: 0/3

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Calculate the probability that a randomly selected person uses Twitter but not Facebook.

$\displaystyle{}$ Expected answer:

$\displaystyle{\frac{ 34 }{ 91 }}$

Score: 0/4.5

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