We notice that 25 and 100 have a common factor of 25:
$$\frac{21}{\cancelto{1}{25}} \cdot \frac{\cancelto{4}{100}}{3}$$
Now, we can multiply the numerators together and the denominators together:
$$\frac{21 \cdot 4}{1 \cdot 3}$$
$$\frac{84}{3}$$
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