18^4/12^5

27/64

Reason:

• 18 = 2 × 3^2 → 18^4 = 2^4 × 3^8
• 12 = 2^2 × 3 → 12^5 = 2^10 × 3^5
• (2^4 × 3^8) / (2^10 × 3^5) = 2^(4-10) × 3^(8-5) = 2^-6 × 3^3 = 27/64
• Decimal: 0.421875