Let's say she made 50 cookies. If she sold 3/5 of them in the morning, that means she sold 3/5 * 50 = 30 cookies. If she sold 1/4 of the remaining 20, she sold 1/4 * 20 = 5 cookies. So she would have 15 remaining.
However, the same logic could be applied if she started with 100 cookies, or 200, or 1000. There are an infinite number of solutions.
