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An upper-layer packet is split into 5 frames, each of which has an 90% chance 
of arriving undamaged. If no error control is done by the data link protocol, 
how many times must the message be sent on average to get the entire thing through?
A. 2.1
B. 1.9
C. 1.7
D. 2.4

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What would be the probability that all reach safe?
$(\frac{9}{10})^{5}$ = 0.59049

Intuitively speaking, the probability of getting a head is $\frac{1}{2}$ then how many attempts would get you a head (provided once tail has come it wont appear again)?
$(\frac{1}{2})^{-1}$, right?

Here also, it would be $\frac{1}{0.59049}$ = 1.69350... ie roughly 1.7

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