Could I suggest a change in the instructions text? It says "Replace the letters in this equation", so I didn't realize that I had to replace the letters in the inequality as well for it to be marked correct.
I wasted time looking for a mistake I had made in the equation, but there wasn't any...!
My starting post here was actually an error - I decided to put the issue in the bug section. Just copied the link of the puzzle.
I made exactly the same mistake that you did.