Rob has budgeted $77 per month for his phone bill. Line access for calls and SMS texts costs $50, and each GB of data is an additional cost. After paying for access, he has enough money left for exactly 6 GB. He decides he wants to download 2 apps, and the apps cost $4.75 each. Rob also buys a ringtone that costs $2.50. How much more money, in dollars, does he need to pay for 4 GB?
