Step 1: Start with the fraction 21/40.

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

21 ÷ 40 = 0.525

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

0.525 × 100 = 52.5%

Final Answer: 21/40 as a percentage is 52.5%.