Understanding periods in calculations of loan notes and interest rates

Hi,

Hope this finds you well. I’m trying to understand how we arrive at the number of periods to be used in a calculation. A question i am looking at goes:

The financial manager of Gaddes Co’s pension fund is reviewing strategy regarding the fund. Over 60% of the fund is invested in fixed rate long-term loan notes. Interest rates are expected to be quite volatile for the next few years. It is currently June 20X3.
Among the pension fund’s current investments are two AAA rated loan notes:

(1) Zero coupon June 20Y8
(2) 12% Gilt June 20Y8 (interest is payable semi-annually)
The current annual redemption yield (yield to maturity) on both loan notes is 6%. The semi-annual yield may be assumed to be 3%. Both loan notes have a nominal value and redemption value of $100.

Required:
(i) Estimate the market price and percentage change from their current value of each of the loan notes if annual interest rates (yields):
(a) increase by 1%;
(b) decrease by 1%.

The changes in interest rates may be assumed to be parallel shifts in the yield curve (yield changes by an equal amount at all points of the yield curve).

The answer for the Zero coupon June 20Y8 shows:

“Current market prices

(1) Zero coupon ?$100/(1.06)^15? = $41.73”

I know how to calculate this part. What i don’t know is how they identified that there would be 15 periods.

Your help is greatly appreciated sirs. And sorry for the length, just needed to give you the details in one scoop.