Question 140285: A computer is infected with the Sasser virus. Assume that it infects 20 other computers within 5 minutes; and that these PCs and servers each infect 20 more machines within another five minutes, etc. How long until 100 million computers are infected?
I need help setting up the equation. Thank you.
Answer by stanbon(75887)   (Show Source):
You can put this solution on YOUR website! Start with 1
after 5 minutes add 20 to get 21
after 5 minutes add 21*20
after 5 minutes add 21*20^2
after 5 minutes add 21*20^3
etc.
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You want:
1 + 21(20)^n = 100 million for n=0,1,2,3...
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21(20)^n is a geometric series with a(1) = 21 and r = 20
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The sum of n terms is a(1)[(1-r^n)/(1-r)]
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Hope this helps.
Cheers,
Stan H.
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