maths problem for DS

[chezlaw]

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Could you post some problems for skalansky to break down into simple logic?

Skalansky flexes his math logic alot so i think it would be interesting to see if he approaches things as uniquely as he makes it sounds like he does.

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here's a very simple maths problem, so simple a kid can solve it. (so simple there's a danger DS may solve it with recourse to nothing but simple logic). Hopefully it will illustrate the point.

What's the sum of the first n natural numbers? i.e

1+2+3+....+n =