At a lemonade stand, pink lemonade costs $2.50 and sweet tea lemonade costs $4. In one morning, the stand sells 21 drinks for $67.50. How many pink lemonades were sold?

lemonade = 11

Step-by-step explanation:

let p = pink lemonade

let t = sweet tea

create a system of equations -- one with quantity, one with associate costs

p + t = 21      

2.5p + 4t = 67.5

substitution method:  let t = 21 - p

2.5p + 4(21 - p) = 67.5

2.5p + 84 - 4p = 67.5

-1.5 p = -16.5

p = 11

t = 21 - 11

t = 10

linear pair postulates