Math
posted by Jill on .
The present value of an annuity due of $400 payable semiannually is $5600. Interest is computed at 6% compounded semiannually. what are the number of payments? What formula should I use for this? Thanks!

sub into your Present Value formula
PV = paym( 1  (1+i)^n)/i
5600 = 400 (1  1.03^n)/.03
.42 = 1  1.03^n
1.03^n = .58
take log of both sides
log(1.03^n) = log .58
n(log 1.03) = log .58
n = log .58/log 1.03
you do the buttonpushing, I got 18.4 payments
There will be 18 full payments of $400 plus a partial payment 
Thanks so much. That was my answer but that formula in my book was for ordinary annuities and I wasn't sure if it applied to annuities DUE also.

I did not catch the "due" part
so I would change it to
5600 = 400 + 400 (1  1.03^n)/.03 , one 400 plus n payments
5200 = 400 (1  1.03^n)/.03
.39 = 1  1.03^n
1.03^n = .61
.
n = log .61/log 1.03
n = 16.722
n = 16.7
So in addition to the first 400 payment we need 16 more full payments, plus a partial payment