What you must keep in mind is that there are 4 jeans in total, which represents 100%
When she bought two jeans, we have the following information:
If both of them are blue:
50% of their jeans is blue now, that is:
x = (50/100) * 4 = 2
2 jeans are blue.
This means that she did NOT have 2 blue jeans before.
Thus:
If they are not blue:
We have 4 jeans that are not blue.
Then, 0% of her jeans is blue now
answer:
0% percentage of her jeans is blue now