Question 37330
The computer viral growth is an exponential function.  Call it N(t), where N is the number of infected computers and t is the time in minutes.  Thus,
N(t) = 20 ^ (t/5)
Now set this equal to 100,000,000 and solve for t, using logs...
20 ^ (t/5) = 100000000
log [20 ^ (t/5)] = log (100000000)
(t/5)log (20) = 8
t = 40 / log 20  or about 30.74 minutes