GPT-5.5 found a solution only after assuming that you're allowed to concatenate numbers together e.g. 8 7 becomes 87 (it complained at first that it was "under-specified") - using Python it brute-forced a solution (actually finding 13): https://chatgpt.com/share/6a1db54f-7ab8-8333-9218-86a469c284...

Are you sure this is 4th grade level?

I questioned OP's "there is an answer online" claim so I checked and the only source I could find of the original question was a 5th grade Russian school for mathematics.

https://aistudio.google.com/app/prompts?state=%7B%22ids%22:%...