If \(\left[ {\frac{3}{4} + \left\{ {2\frac{1}{2} - \left( {\frac{1}{2}of\frac{1}{{42}}} \right)} \right\}} \right] \div \left[ {\frac{1}{3} + \left\{ {4\frac{3}{4} - \left( {3\frac{1}{6} - 2\frac{1}{3}} \right)} \right\}} \right] = 1 - \frac{k}{{21}}\) then the value of k is:
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