James S. answered 09/26/23
Let m1 be the number of miles driven on the first plan, and c1 be the total cost of the first plan.
Then
c1 = 0.3×m1 + 50
Let m2 be the number of miles driven on the second plan, and c2 be the total cost of the second plan.
Then
c2 = 0.8×m2
To find out the amount of driving ( in other words, when the mileage on both plans) results in equal total costs, set the costs equal and use the cost expressions to solve for the mileage.
Since m1 will equal m2 when c1 = c2, let's now just use m for the mileage.
0.8m = 0.3m + 50
Subtract .3m from both sides:
0.5m = 50
Divide both sides by 0.5
m = 100
Checking,
0.8 × 100 = 80
and
0.3×100 + 50 = 30 + 50 = 80
So, our answer of 100 miles does check out, and as a bonus, we also know that the cost would be $80.