Sum based on filtered columns not rows

[phillip87]

I have a spreadsheet that goes across the screen, this is grouped by month and I need the totals to change when each is grouped or ungrouped.

i.e. B4 shows 1720 which is a sum of data from E4:Z4, as E4:Z4 is grouped the sum of 1720 needs to change and reflect this.

I can only seem to find solutions for data that goes down the sheet grouping by row.