Answer :
Using the concepts of complex numbers, we got that Kristin requires n days at least to get one copy of each of the n card types.
Complex numbers are those numbers that are expressed in the form of a + ib where, a, b are real numbers and ‘i’ is an imaginary number known as “iota”. The value of i = (√-1). Like, 2+3i is an complex number, where 2 is a real number (Re) and 3i is a imaginary number (Im).
So, let suppose Z be the number of days until she collects at least one copy of each the n card types. For each i let [tex]Y_i[/tex] be the days she gets her ith type of card. In particular, Y₁=1 and
Y₁<Y₂<.........Yₙ=Z.
Let [tex]Z_{i}[/tex]=[tex]Y_i-Y_(i-1)[/tex].
Then Z=Z₁+Z₂+Z₃+.........Zₙ.
Identifying each [tex]Z_i[/tex] as a geometric progression and use this to find E(|Z|).
This will tell that she requires at least n days.
Hence, the expected number of days so that Kristin collected at least one copy of the n card types is n days.
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