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You can do that on any calculator. Just keep dividing both numerator and denominator until it reaches a point where they cannot be simplified further on.

However, to answer your question,

$\frac{84}{16} = \frac{42}{8} = \frac{21}{4}$.

So our answer would be $\boxed{\frac{21}{4}}$. If you wanted it in a mixed number, it would be $\boxed{5\frac{1}{4}}$.

Without any difficult culculations you can transform your significant number into simple 21/4.