The fraction of smarties that are brown is $\frac{2}{26}=\frac{2\times 1}{2\times 13}=\frac{1}{13}$.
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Adding on a little: In general, if you have "$some$ out of $total$ are brown," the fraction equivalent is: $$\frac{some}{total}$$
– apnortonJan 27 '13 at 22:27