When you would use the general addition rule or the special addition rule?

Ingredient c: 1/4 cup for 2/3 serving or ingredient d: 1/3 cup for 3/4 serving which unit rate is smaller

Atruck can be rented from company a $50 a day plus $0.60 per mile. company b charges $20 a day plus $0.80 per mile to rent the same truck. find the number of miles in a day at which the rental costs for company a and company b are the same

For the sequence [tex]a_{n} = 2n/(n+1)[/tex], what is the value of [tex]a_{10}[/tex]