Strategy: This is a great question. In my podcast episode 1001, I had several people write in to say that they regularly used this method to add horizontal subtotals. Although it is a lot of steps, if you use shortcut keys, it is actually fast.
1. Select the original data with Ctrl+*
2. Go a few rows below the data. Paste with Alt+E+S+E+Enter.
3. Alt+D+B to display the Subtotals dialog. Click OK.
4. Ctrl+C to copy the vertical data set with the subtotals.
5. Select cell A1.
6. Paste Transpose with Alt+E+S+E+Enter.
7. Fix the column widths with Alt+O+C+A.
Figure 683 Horizontal subtotals.
8. Delete the temporary table at the bottom.
9. Optionally, select columns B:D and choose Data, Group.
10. Select columns F:H and press F4 to re-do the group command.