I can sell 25 cookies for $42.50. Write an equation relating the total cost, c, to the number of cookies, n.

25n = 42.50c

Explanation:

25 cookie sells for $42.50. Therefore, you need to add the variable n for the number of cookies on one side. Thus, $42.50 is worth 25 cookies, so you would add c for the cost on the other side, and you get 25n = 42.50c.

Jun.       Jul.                 Agu.                 sept.                  oct.                     
20 + 2 => 22 + 2.2 = > 24.2 + 2.42 => 26.62 + 2.662 => 29.282 + 2.9282=>
Nov.                           Dec
32.2102 + 3.22102 =>  35.43122 + 3.543122 ===>>> $38.974342 (rounded: $39)