Step 1: Start with the fraction 21/4.

Step 2: To convert a fraction into a percentage, divide the numerator by the denominator.

21 ÷ 4 = 5.25

Step 3: Multiply the result by 100 to get the percentage.

5.25 × 100 = 525.0%

Final Answer: 21/4 as a percentage is 525.0%.