Apparently this maths problem is doing the rounds. It didn't seem that challenging to me, though I can't comment on how difficult it would be for 10-year-olds.
I post it here mainly for the note at the end, reminding us that "education is not just about rote learning but also about critical thinking and problem-solving." I'm not entirely sure about the phrasing of this claim, since it seems to imply that education is partly about rote learning. Maybe that's truer at lower levels though.