Answer
64.91 amu
Work Step by Step
Atomic mass = (mass I1)(Abundance I1)+(mass I2)(abundance I2)
So, mass I2 = \frac{Atomic mass - (mass I1)(Abundance I1)}{Abundance I2}
=\frac{63.546-(62.939\times0.6917)}{1-0.6917}
= 64.91 amu
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