>The numbers do not fit 320,000 * 24,000 is . . .

Here's a mental math flow for this estimate:

      320,000 * 24,000  
     =(320 * 24) million (let's deal with the easy zeros first)
    ~=(300 * 20) million (underestimate)
     =(600 * 10) million
     = 6 billion (very roughly).

Another flow:

      (320 * 24) million 
    ~=(320 * 25) million (overestimate)
     =(80 * 100) million
     =8 billion.

So we have a number between 6 and 8 billion, 7 sounds about right.