Step 1: Identify the number of decimal places

The decimal 3.9909 has 4 decimal places.

Step 2: Multiply by the appropriate power of 10

Multiply by 10⁴ = 10000 to eliminate the decimal point.

3.9909 × 10000 = 39909

Step 3: Write as a fraction

Step 4: Simplify the fraction

Find the greatest common divisor (GCD) of 39909 and 10000.

GCD(39909, 10000) = 1

Divide both the numerator and denominator by the GCD: