Let G stand for the number of GB billed, so the total bill is $0.25G + $45 flat rate charge = $65 total bill.
Now we will solve for G. $0.25G + $45 flat rate - $45 = $65 -$45. This simplifies to $0.25G = $20
$0.25G/$0.25 = $20/$0.25
G = 80 GB were charged on this bill.
#### 80