I have 46 records in my database.

On my report sheet, I can only display 15 records at a time. I want to have a sub total for each page of the 15 records.

How do I do that?

Hmm. You can sort and summarize by a field equal to Div( Get( RecordNumber ) - 1, 15 ), but then you must sort by this field only.

A better way is probably that: make an unstored calculated number Previous Records with 15 repetitions:

Let( [

 record number =  Get( RecordNumber );

 relative record number =  Mod( record number - 1; 15 );

 repetition = Get( CalculationRepetitionNumber ) ];

  If( repetition > relative record number + 1; 0; 

    GetNthRecord( Extend( Value ); record number - ( repetition - 1 ) ) ) )

Add yet another calculation Sum to sum these values:

Sum( Previous Values )

Place the calculation in the footer and you're done. It handles the last incomplete page too.