Answer
Mass of $^{185}Re$= 185.0 amu
Work Step by Step
Let m be the molar mass of $^{185}Re$:
$186.207 = 0.6206*186.956+(1-0.6206)m$
$m=\frac{186.207-0.6206*186.956}{(1-0.6206}=185.0$ amu
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