Let M = #miles driven
Plan 1: .50M + 70 = total cost
Plan 2 .70M = total cost
To know how far she would drive to have an equal cost for either plan just set the equations equal to each other
.70M = .50M + 70
subtract .50M from both sides
.20M = 70
M = 70/.20 = 350 miles
Check the answser:
Plan 1: .50(350) + 70 = 175 + 70 = $245
Plan 2: .7(350) = $245
they are equal so our answer checks as correct