def years_to_double(money, rate):
    '''Return how many years it takes for an investment to double at a
    given interest rate.'''

    years = 0

    # Each year until the investment has doubled, add 
    # interest for the year.
    while money != (money * 2): 
        years += 1
        money += rate * money
    
    return years

if __name__ == "__main__":
    
    # How many years will it take to turn 10 dollars into 20 dollars
    # if 10% interest is added every year?
    interest = 0.1
    investment = 10
    years = years_to_double(investment, interest)
    
    print "At an interest rate of " + str(interest),
    print "your money will double in " + str(years) + " years."