How long will it take money to double if it is invested at 5% compounded annually? ..... I am pretty sure I have the right answer but the back of the text says something else. Just want to confirm that I'm right and it's wrong! lol

Question

debbieg · Answer

I get 14.2 years.

2=(1+0.05)^t
ln2=t*ln(1.05)
t=ln2/(ln1.05)=14.2

Am I missing something? Book says 15 years.

anonymous · Answer

i don't think you're wrong.

my ideas:

-maybe they are rounding up since it is unlikely to be 15.000 years. 

-unlikely: maybe they used the "rule of 72" but 72/5 = 14.4. still not justifiable to round up.

but this is how this problem is meant to be done

debbieg · Answer

Thanks. Certainly not the first time an answer in the back of the book is wrong, but I wanted to be sure I wasn't making some dumb mistake. I don't think it's rounding, because the other part of the same problem is to find the time under continuous compounding, and there they have 13.9 (which I get too).

anonymous · Answer

lol i hate when that happens :P

i've wasted so much time with wrong textbook answers