Hi Ifonly....
I'm just a Mum, but given you're a teacher you'll be better able to judge if this is a good approach:
This kind of approach (expecting a set amount of work - regardless of difficulty) to get done can lead to someone equating 1+1 problems to 3369 + 67, which involves carrying over, more time for calculation and more time to check answer is correct.
So the problem may be that the teacher is equating lower level work to higher level work in terms of time, and needs to review that a bit.
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If the problem is that your DD is losing interest - well this is actually an issue.
You don't want her losing interest half-way through her KS2 Maths SATs. 'But I've done 5 problems, I don't do more' won't cut it really, will it?
So I think you have to investigate whether your DD is refusing to do more than a few problems. Now this can be for all sorts of reasons: too easy, too complicated, everyone else is done so she wants to stop too, she can do it but doesn't really like maths, etc...
It may be that the table (maths group) is always waiting for her to finish - and this is creating tension.
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I think the approach I would take (and I'm no saint and probably would say the wrong thing and get everyone's back up) - but the approach I'd try is to say you don't really understand what the problem is with your DD not finishing work - do they have a theory? Hopefully they'll expand on what they think the problem is and a solution will present itself.
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Finally - and I'm not trying to pick a fight - but you clearly have strong views that you only need to work a maths concept 5 times and you learn it - you said above '20 doesn't help'. This could well be what is influencing your DD's attitude.
Everyone is different - but I can assure you for my DD1, twenty (even 50) is required before she really gets a concept so well she can do it without thinking.
If the school desire her to work through 30 multiplication problems, for example, and she's packing an attitude about only wanting to do 5 and no more - there will be conflict. Perhaps the solution is to encourage her to think about it as a speed contest. If it is in fact very easy for her, and she knows it all, she might find she enjoys finishing first (with all answers correct) to prove how well she understands the concept.
HTH