\textbf{215.} A chemical substance contains three atoms of chromium in its chemical formula. If $31.2\%$ of this substance is chromium, what is the molar mass of this substance? $(Cr = 52 : \text{g.mol}^{-1})$
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(1) $166.7$ \hfill (2) $250$ \hfill (3) $333.3$ \hfill (4) $500$
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