2.1.7 In a series of 40 payments the first 10 payments are 10 each, the second 10 payments are 20 each, the third ten payments are 30 each, and the final 10 payments are 40 each. The payments are equally spaced and the interest rate is 5% per payment period.

*2.1.23 For the series of part (a) of Exercise 2.1.7, find the present value one payment period before the first payment. Show that this present value can be written as