what is the national maths challenge?

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ds seems to be in a team

"It's a team thing if you get to a high enough level, I think."

One of the purpose of the challenges is to select a team to represent UK at the International Maths Olympiad, so yes it definitely is a team thing if you get high enough! The individual challenges do stay individual in the higher rounds though.

I am thinking if your ds did it yesterday NC then it can't have been the JMC (Junior maths challenge) which is on 29th, unless it was a practice run.

Yes, it might have been a practice run for the JMC. (next week as snorkie says).

Or it could have been something in-house which they are going to use to select students to represent the school at the Team event.