Formula:

f_−3dB = ¹⁄_2πRC  ≈  ^0.159⁄_RC

Useful shortcuts:

• 2π ≈ 6.28 ≈ 6.3
• 1/(2π) ≈ 0.159 ≈ 1/6.3
• R = 1 kΩ, C = 1 µF → f ≈ 159 Hz
• R = 10 kΩ, C = 1 µF → f ≈ 15.9 Hz
• R = 1 kΩ, C = 10 nF → f ≈ 15.9 kHz

Answers within ±20% are accepted — this is a game about estimation, not exact computation.