Step 1: Start with the fraction 21/5.

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

21 ÷ 5 = 4.2

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

4.2 × 100 = 420.0%

Final Answer: 21/5 as a percentage is 420.0%.