中三數學問題(indices)

Let x be the initial amount.
After n years, the deposit will increased to x(+10%)n
In order to be triple, this amount must be more than 3x
x(1.1)n>3x
1.1n>3
nlog(1.1)>=log(3)
n>=11.5
Minimum for n is 12


2009-07-17 09:18:54 補充：
After n years, the deposit will increased to x(1+10%)n

2009-07-17 10:33:52 補充：
After n years, the deposit will increased to x(1+10%)^n

Let Joey deposits $y.
years needed be x.
y(1+10%)^x = 3y
1.1^x = 3
x = 11.52670461.....
at least 11.52670461 years are needed to triple money

(I don't know how to do the last step, so I try the number randomly.)