Was that part of a bigger prompt?
Flash 3.5 fails exactly like in your sample: https://gemini.google.com/share/97521a8752d9
but Flash 3.1 Lite initially fails, but then corrects itself: https://gemini.google.com/share/dc0889ec85ba
Flash 3.5 fails exactly like in your sample: https://gemini.google.com/share/97521a8752d9
but Flash 3.1 Lite initially fails, but then corrects itself: https://gemini.google.com/share/dc0889ec85ba
Are you using the flash models? Reasoning models or extended thinking will change the result.
GPT 5.5. Instant shows the same error. If the given prompt isn't working, you can also try "300+140=460 is this correct?". I suspect that leading with the equation may be part of the issue, but haven't tested much.