What is happening here?
Puzzle tasks like the Magic Triangle or number riddles have no obvious solution strategy. Many children are used to giving quick answers. When that does not work, frustration sets in. This is not a sign of a lack of intelligence, but of being unpracticed at putting up with uncertainty.
Praise the path, not the result
Instead of "Correct!", try saying: "You tried that systematically, that is exactly the right strategy." Or: "You noticed that your first try did not work, and you kept going. That is mathematics."
Children who are praised for persistence and their thinking give up less quickly on hard problems than children who are only praised for correct answers.
When a break helps
When frustration shows, a short break is almost always better than pushing through. After five minutes away, a new idea often comes to mind. Say: "Let us stop for a moment and look again in a bit." That is not giving up, it is a problem-solving strategy.
Helping without solving the task
Asking questions helps more than giving answers: "What do you already know?", "Have you tried all the numbers yet?", "What happens if you start with the smallest number?" The child works it out themselves and the sense of success stays theirs.