Sample space S = {H, T} = {heads, tails}
Date of birth: January 1, 1991 --> day of birthday = 1
Let X = indicator random variable corresponding to an event in the sample space S.
Pr{coin toss results in heads} = Pr{X = 1} = 1/day_of_birthday = 1/1 = 1
Let x = number of coin tosses. E[X] = sum(x*Pr{X = x}) = x
The amount that Bob owes me = $5*E[X = 1] + $1*E[X = 0]
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Edited: 2017-08-30)
Sample space S = {H, T} = {heads, tails}
Date of birth: January 1, 1991 --> day of birthday = 1
Let X = indicator random variable corresponding to an event in the sample space S.
Pr{coin toss results in heads} = Pr{X = 1} = 1/day_of_birthday = 1/1 = 1
Let x = number of coin tosses. E[X] = sum(x*Pr{X = x}) = x
The amount that Bob owes me = $5*E[X = 1] + $1*E[X = 0]