The population of humorville is 8500 people. In this town jokes travel fast. In one hour, each person who hears a joke tells three other people who have not herd it, and tells no one else. Last friday, a visitor from out of town told the police cheif a new joke at 10:00a.m. How lomg did it take for the joke to travel through Humorville and for everyone to hear it??

Answer 1

now 1 person knows the joke -- 1
after 1 hour: 3^1 new people will know the joke
after 2 hours 3^2 new people will know the joke
after n hours 3^n new people will know the joke

so 1 + 3 + 3^2 + 3^3 + ... 3^n = 8500
geometric series, a = 1 , r = 3 , n = ?
S(n) = a(r^n - 1)/(r-1)
8500 = 1(3^n - 1)/2
17000 = 3^n - 1
3^n = 17001
log(3^n) = log17001
nlog3 = log 17001
n = log 17001/ log 3 = 8.86 or appr 9 hours