Compound interest question?

Question

How long will it take $1000 to double at 4% 
interest compounded annually? 

Using compound interest formula, could you please explain?

Answer 1

The principal is $1000 .

Interest rate 4% .

Interest is compounded annually .

Amount of money accumulated after t years is A

Formula for compound interest is  

P = principal amount (the initial amount you borrow or deposit) = $1000 

r  = annual rate of interest (as a decimal) = 0.04

t  = number of years the amount is deposited 

A = amount of money accumulated after n years, including interest. = $2000

n  =  number of times the interest is compounded per year = once .

So the time taken to double the principle is 17.67 years .