ChatGPT-4 can solve it them. Even using GPT-5 from the API gives the correct result. Something with the ChatGPT frontend prompt seems to be the issue.

Example:

Prompt: Solve 5.9=x+5.11

Answer:

To solve the equation:

5.9 = x + 5.11

Subtract 5.11 from both sides:

5.9 - 5.11 = x

-0.21 = x

Final answer:

\boxed{x = -0.21}