Question

There are two plans for renting a car. Plan A charges $10 per day plus a $50 one-time fee. Plan B charges $15 per day plus a $30 one-time fee. How many days must you rent the car for Plan A to be a better deal than Plan B?

Answer 1

we need

50+10d < 30+15d

so, ...

Answer 2

50 + 10d < 30 + 15d
-10d < - 10d
50 < 30 + 5d
-30 -30
20 < 5d
/5 < 5
4 < d

Please tell me if this is the right answer.

Answer 3

Looks good to me.
If more than 4 days of rental, A is the better deal.

At d=4, A=B=90
After that, A grows more slowly than B.