Carcass wrote:
If we apply 8% annual interest, compounded quarterly, then we apply one quarter of the interest (or 2% interest) four times per year. That is, in one year, we will multiply the value of our investment by 1.02 four times, or in other words, by (1.02)^4. So, if we invest for x years, we will apply 2% interest 4x times, so will multiply the value of our initial investment by (1.02)^(4x). Now, we know that the value has increased by a factor of 16, so
(1.02)^(4x) = 16
(1.02^x)^4 = 2^4
1.02^x = 2
My point is that the question is poorly worded. If they state annual interest and x is in terms of years, then it does not matter that our interest rate is compounded quarterly. The question can lead to a lot of confusion if it is actually referring to a convertible quarterly interest rate.