Concept:Understanding percentages and solving for an unknown value.Explanation:Let's say the unknown number is $x$.The problem states that 21% of this number $x$ is equal to 63.We can write this as an equation:$21\% \text{ of } x = 63$To convert the percentage to a fraction, we divide by 100:$\frac{21}{100} \times x = 63$Now, to find $x$, we need to isolate it. We can do this by multiplying both sides of the equation by $\frac{100}{21}$:$x = 63 \times \frac{100}{21}$We can simplify this calculation. Notice that $63$ is $3 \times 21$. So:$x = (3 \times 21) \times \frac{100}{21}$The $21$ in the numerator and the $21$ in the denominator cancel out:$x = 3 \times 100$$x = 300$So, the number is 300.Answer:300