Step 1: Identify the number of decimal places

The decimal 15.421 has 3 decimal places.

Step 2: Multiply by the appropriate power of 10

Multiply by 10³ = 1000 to eliminate the decimal point.

15.421 × 1000 = 15421

Step 3: Write as a fraction

Step 4: Simplify the fraction

Find the greatest common divisor (GCD) of 15421 and 1000.

GCD(15421, 1000) = 1

Divide both the numerator and denominator by the GCD: