To sum every nth column, you can use a formula based on the SUMPRODUCT, MOD, and COLUMN functions.
In the example shown, the formula in L5 is:
=SUMPRODUCT(--(MOD(COLUMN(B5:J5)-COLUMN(B5)+1,K5)=0),B5:J5)
How this...

To get a full set of relative column numbers in a range, you can use an array formula based on the COLUMN function.
In the example shown, the array formula in B4:H4 is:
{=COLUMN(B4:H4)-COLUMN(B4)+1}
On the worksheet...

To locate the position of a value in a 2D array, you can use the SUMPRODUCT function. In the example shown, the formulas used to locate the row and column numbers of the max value in the array are:
=SUMPRODUCT((data=...

To count with repeating numbers (for example: 1111,2222,3333,4444, etc.) you can use a formula based on the ROUNDUP function, with help from either ROW or COLUMN.
In the example shown, the formula in C4 is:
=ROUNDUP...

If you need to dynamically increment a calculation, so that a value automatically increments each time the formula is copied to a new row or column, you can use the ROW() or COLUMN() functions in your formula.
In the...

You can get the last column in a range with a formula based on the COLUMN function.
In the example shown, the formula in cell F5 is:
=MIN(COLUMN(data))+COLUMNS(data)-1
where data is a named range for B5:D10
How...

To convert a column letter to an regular number (e.g. 1, 10, 26, etc.) you can use a formula based on the INDIRECT and COLUMN functions.
In the example shown, the formula in C5 is:
=COLUMN(INDIRECT(B5&"1...

To get the address of the first cell in a named range, you can use the ADDRESS function together with ROW and COLUMN functions.
In the example shown, the formula in G6 is:
=ADDRESS(ROW(data)+ROWS(data)-1,COLUMN(data...

To count rows that contain specific values, you can use an array formula based on the MMULT, TRANSPOSE, COLUMN, and SUM functions. In the example shown, the formula in G5 is:
=SUM(--(MMULT(--(data=90),TRANSPOSE(COLUMN...

To generate a fixed value every N months (for example, a fixed expense every 3 months, a fixed payment every 6 months, etc.) you can use a formula based on the MOD function.
In the example shown, generate a value of 60...