Final answer:
Amy gave away 63 cookies by the time she got to her 5th-period class, represented by the equation C = 75 - 12, where C is the number of cookies given away.
Step-by-step explanation:
To represent Amy's cookies given away, we can use the equation C = 75 - 12, where C represents the number of cookies Amy gave away.
By 5th period, Amy gave away 63 cookies since she started with 75 cookies and was left with 12. Therefore, the statement would be: Amy had given away 63 cookies by the time she got to the 5th period.