MatsT
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 Jun 14 comment Find two closed subsets or real numbers such that $d(A,B)=0$ but $A\cap B=\varnothing$ @kahen Wouldn't 1 be in both your sets assuming 0 is included in the natural numbers? Jan 31 comment Find five positive integers whose reciprocals sum to $1$ From the question it seems clear that the distinct requirement was not given to the 5th graders. Fractions and equations with multiple unknowns is part of the 4-6th grade plan so the problem seems appropriate (though maybe not required for a passing grade). I would expect somewhere between 70-90% to figure out the trivial solution and maybe 20-30% to figure out another solution, thus proving it's not the only one. Less than 1% of 5th graders would probably find a distinct solution. Nov 6 comment Given $f(f(x))$ can we find $f(x)$? I think what he's really asking about is a function that is infinitely differentiable. Jul 6 comment Is '10' a magical number or I am missing something? The alien would still need to know what 'one' and 'zero' is, there's no reason to assume he would use the same words or symbols for these. He's probably using base ☺┼ Jan 31 awarded Supporter