Discrete Mathematics
posted by Joy .
There are two plans for renting a car. Plan A charges $10 per day plus a $50 onetime fee. Plan B charges $15 per day plus a $30 onetime fee. How many days must you rent the car for Plan A to be a better deal than Plan B?

we need
50+10d < 30+15d
so, ... 
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. 
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.