Autofill referencing every nth cell

Hi,

I am having trouble using autofill to reference every nth cell.

For example, for N=2, I want

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For N=5, I want

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The values are then summed and multiplied by a constant and then N.
For N=2, SUM(column)*0.035/3*2
For N=5, SUM(column)*0.035/3*5

Attached is the .xlsx of a small representative data of what I am trying to achieve.

N.B.: I have asked a similar question and was answered by user Glenn Kennedy prior in a thread called "Autofill referencing non-adjacent cells" (unable to post URL link). He used an array formula which may be applicable to this case. I have been unsuccessful in modifying the formula for this new scenario although it may speed up in reaching a solution to my problem. Once again, appreciate any help given!

Try this formula in C3
drag to right an down

Formula:

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Originally Posted by salim ali

Try this formula in C3
drag to right an down

Formula:

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Hi Salim,

Thank you for the help. Dragging does produce the correct values.
It is possible to now use the SUM function to add those values up?
For example, for N=5, adding every 5th value in column G. I have highlighted this in orange.

Originally Posted by Bo_Ry

Please try at I2
=IF(MOD(ROWS(I$3:I3)-1,I$2),"",$B2+INDEX($B3:$B24,I$2)+($B2+INDEX($B3:$B24,I$2))^0.5)

Hi Bo_Ry, appreciate your input. I am afraid I do not understand how your formula works but it does produce the correct values!
May I please ask how I can modify your formula for N=3 or N=10 for example?

Thank you.

Hi everyone,

Just wanted to say I was able to solve my issue. I used the function SUMPRODUCT to add up the values every nth row for those who might be wondering.

Thank you to all who replied, much appreciated for the help.