Problem 1
PMT = 505.76 = monthly payment
k = monthly interest rate in decimal form
k = 0.043/12 = 0.003583333 (approximate)
n = 5*12 = 60 months
PVOA = present value of ordinary annuity
PVOA = PMT * ( 1 - (1+k)^(-n) )/k
PVOA = 505.76 * ( 1 - (1+0.003583333)^(-60) )/0.003583333
PVOA = 27,261.436358296
When rounding to the nearest dollar, we get $27,261
Your teacher made a mistake in choosing the formula. S/he mixed up present value ordinary annuity with annuity due. The (1+k) portion at the end is ignored. I rewrote the 1/( (1+k)^n ) sub-portion as (1+k)^(-n) to avoid a bit of clutter.
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To type this into excel we will write
=PV(0.043/12,5*12,505.76,0,0)
That will produce the result of -27,261.44. The negative is to indicate a cash outflow.
Don't forget about the equal sign up front when writing excel formulas.
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Problem 2
L = loan amount = 23099
k = interest rate per month = 0.051/12 = 0.00425 exactly
n = number of months = 5*12 = 60 months
PMT = monthly payment
PMT = (Lk)/(1 - (1 + k)^(-n) )
This formula is the result of solving PVOA = PMT * ( 1 - (1+k)^(-n) )/k for "PMT". The PVOA value is the loan amount in this case.
Let's plug in the values mentioned
PMT = (Lk)/(1 - (1 + k)^(-n) )
PMT = (23099*0.00425)/(1 - (1 + 0.00425)^(-60) )
PMT = 436.965684557303
PMT = 437 when rounding to the nearest whole number
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To do this in excel, we type in
=PMT(0.051/12,5*12,23099,0,0)
The output should be -436.97 which rounds to -437.
The value is negative to represent a cash outflow, but your teacher mentions to post the answer as a positive value.