A sum of money at compound interest becomes double of itself in 15 years. Find the rate of interest. I am unable to solve this because there are two unknown values(principle and rate per annum).

Question

anonymous · Answer

principle values is unimportant.
double means double, so if you start with $100 you end with $200 and if you start with $1,000,000 you end with $2,000,000 
what i don't is how frequently it was compounded

anonymous · Answer

You should take it as annually(you mean half yearly, quarterly and so on don't you)?

anonymous · Answer

ok annually
set 
$$(1+r)^{15}=2$$ and solve for $t$

anonymous · Answer

I don't understand. And where is t?

anonymous · Answer

I only know one formula regarding compound interest:
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