555

Do you mean:

• (a) the number 555 (math), or
• (b) the 555 timer IC (electronics), or something else?

If you want a quick start for both:

1. Math: 555 as a number

• Prime factorization: 555 = 3 × 5 × 37 (all distinct)
• Divisors: 1, 3, 5, 15, 37, 111, 185, 555 (8 divisors)
• Quick checks: divisible by 3 and 5; odd
• Quick practice ideas: gcd(555, 1000) = 5; 555 mod 37 = 0; sum of digits is 15

2. 555 timer IC (electronics)

• Pinout (8-pin): 1 GND, 2 Trigger, 3 Output, 4 Reset, 5 Control, 6 Threshold, 7 Discharge, 8 Vcc
• Modes: astable (self-oscillating), monostable (one-shot), bistable
• Astable basic formulas:
  • f ≈ 1.44 / [(R1 + 2R2) C]
  • Duty cycle D ≈ (R1 + R2) / (R1 + 2R2)
• Monostable (one-shot) basics:
  • Pulse width t ≈ 1.1 × R × C
• Example: with R1 = 1 kΩ, R2 = 10 kΩ, C = 1 μF
  • f ≈ 1.44 / [(1k + 2×10k) × 1 μF] ≈ 68 Hz
  • D ≈ (1k + 10k) / (1k + 2×10k) ≈ 52%
• Typical supply: 5–15 V

Tell me which context you want, and I can go deeper or give you specific problems to practice.