Let x = cookies
For first rate:
1.25x < 12
For second rate:
1.00x = 12
For each additional cookie:
0.75x > 12
Then, answer is:
$1.25x when x < 12
$12.00 + $0.75(x - 12) when x >= 12
$12.00 + $0.75x when x > 12
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