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What will be the number of stereoisomers of $\ce{CH3CH(OH)CH(OH)CH(OH)CH3}?$

By formula $2^n - 1 + 2^n + 1/2$ for $n$ being the odd number of chiral centres, it's $6;$ four optically active and two optically inactive.

Optically active when RRS, SSR, RR, SS, but I am not getting how we get two optically inactive.

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