21:8?

Could you clarify what you mean by 21:8? Here are the common interpretations:

• As a ratio: 21:8 is already in simplest form (gcd(21,8) = 1). If you want a decimal, 21/8 = 2.625; as a mixed number, 2 5/8; as a percentage, 262.5%.
• As a time: 21:08 in 24-hour time is 9:08 PM in 12-hour time.
• If you meant something else (a fraction, a proportion, etc.), please share the exact problem and I’ll help solve it.