For this case we can set uo an equation given by:
[tex]y=10x+20[/tex]Where y represent the final cost. x the number of Gig used and for this case we can set up the following equation:
[tex]60=10x+20[/tex]And solving for x we got:
[tex]x=\frac{60-20}{10}=\frac{40}{10}=4[/tex]And the final answer for this case woudl be 4 Gig of data used