To write $\frac{21}{24}$ as a decimal, first simplify the fraction:

$\frac{21}{24} = \frac{7}{8}$

Now, divide $7 \div 8$ to convert it into a decimal:

$7 \div 8 = 0.875$

Thus, $\frac{21}{24}$ as a decimal is $0.875$.

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Would you like further explanation or details? Here are some related questions:

1. How do you simplify fractions to their lowest terms?
2. Can all fractions be expressed as terminating decimals?
3. What is the decimal representation of $\frac{5}{8}$?
4. How do you convert repeating decimals back into fractions?
5. What is the difference between terminating and repeating decimals?

Tip: Simplifying fractions first can make decimal conversion easier and help identify whether the decimal will terminate or repeat.