I need to calculate how big amount of interest income is amortizing. I have
sales of the each period, amort rate (how much amortizes in 1-st ,2-nd and so
on period after sales, and the Yield applied to each period sales.
The problem is, that I could not figure out, how to build the sumproduct
function so, that on each next period it would take in account, that there is
new volume that started to amortize and the others have one more amortization
rate to be taken in account.

B C D E F
3 New Sales 195 192 146 394 97 596 1 076 343
4 New Sales Yield 7,25% 7,22% 7,19% 7,26%
5 Amortization 1,12% 1,13% 1,13% 1,14%
6 Amortized interest Income 159 437 795 2 031

In simple formulas:
C6=$B3*$B4*SUM($B5:B5)
D6=$B3*$B4*SUM($B5:C5)+$C3*$C4*SUM($B5:B5)
E6=$B3*$B4*SUM($B5:D5)+$C3*$C4*SUM($B5:C5)+$D3*$D4 *SUM($B5:B5)F6=$B3*$B4*SUM($B5:E5)+$C3*$C4*SUM($B5 :D5)+$D3*$D4*SUM($B5:C5)+$E3*$E4*SUM($B5:B5)
and so on

Can somebody help please?