= Solution
The <arithmetic mean> is $\bar x=n^{-1}\sum_{i=1}^n x_i$, the total measured concentration divided by the number of observations. The <median> is the central value of the sorted observations, or an interpolation between the two central values under the chosen even-sample convention. It is the 50th <percentile>.
The table's “75th quartile” means the 75th <percentile>, or upper <quartile> $Q_3$, with approximately three quarters of observations at or below it. Its “25th quartile” means the 25th <percentile>, or lower <quartile> $Q_1$. There are four quarters of a distribution, rather than 75 different quartiles. <Quantile> interpolation rules can differ for a finite sample; their intended population interpretation is unchanged.
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