555

Do you mean:

• math: the number 555 (factorization, divisors, etc.), or
• electronics: the 555 timer IC and its circuits (monostable/astable)?

If math, here are quick facts:

• Prime factorization: 555 = 3 × 5 × 37
• Divisors: 1, 3, 5, 15, 37, 111, 185, 555
• Number of divisors: 8
• Sum of divisors: 912
• Euler’s totient: φ(555) = 288
• Product of divisors: 555^4

If you want a quick practice problem, I can give one. If you want 555 timer basics, I can summarize formulas for monostable/astable modes. Which would you prefer?