Step 1: Identify the number of decimal places

The decimal 21.561 has 3 decimal places.

Step 2: Multiply by the appropriate power of 10

Multiply by 10³ = 1000 to eliminate the decimal point.

21.561 × 1000 = 21561

Step 3: Write as a fraction

Step 4: Simplify the fraction

Find the greatest common divisor (GCD) of 21561 and 1000.

GCD(21561, 1000) = 1

Divide both the numerator and denominator by the GCD: