Data:
8% commisions on the first $8000
12% commisions on a sell over $8000
Total commisions: $1480
You have the next equation for the situation: x is the sells over the $8000

Total sells:

If the total commisions are $1480:

Then, as the sells over $8000 are $7000, the total sell are the sum of 8000 and 7000:

Total sells: $15000