Suppose the 8% coupon, 30-yeat maturity bond sells for $1,150 and is callable in 10 years at a call price of $1,100. Its yield to maturity and yield to call would be calculated using the following inputs:

	Yield To Call	Yield To Maturity
Coupon (PMT)	$40	$40
Number of Semiannual Periods (n)	40 periods	60 periods
FV (Face or Par Value)	$1,100	$1,000
PV (Present Value)	$1,150	$1,150

Yield To Call: On the financial calculator, input n = 20, PMT = 40, FV = 1,100, PV = -1,150, CPT I/Y = 3.32% or 6.64% annual bond equivalent yield

Yield To Maturity: On the financial calculator, input n = 60, PMT = 40, FV = 1,00, PV = -1,150, CPT I/Y = 3.41% or 6.82% annual bond equivalent yield

The only thing that I would like to know is for yield to call, I understand that in order to find the number of semiannual periods, you take 30 years to maturity - 10 years at call = 20 years x 2 = 40 periods. Why does 40 periods have to be divided by 2 in order to get 20 periods? Why can't the 40 periods be left alone like 60 periods for YTM? Please explain.