For non-negative real numbers $x_{1},\: x_{2},\: ...,\: x_{n}$, which satisfy $x_{1}+x_{2}+...+x_{n}=1$, find the largest possible value of $\displaystyle \sum_{j=1}^{n}(x_{j}^{4}-x_{j}^{5})$.

Give your answer to 3 significant figures.

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