I don't think your answer is correct. When you use your answer and subtract 90, you're only left with 1/2 of what you had where you're supposed to have more than 1/3 minimum . You can use this method to check your answer.

It says that after using $90 for the skirt, she is left with 1/4 of her money. You can interpret it as 2/3 left of her total sum if you only subtract the dress. From there is just solving the equation.