Step 1: Start with the fraction 21/7.

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

21 ÷ 7 = 3.0

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

3.0 × 100 = 300.0%

Final Answer: 21/7 as a percentage is 300.0%.