555

Do you mean one of these, or something else?

• Math: 555 as a number — prime factorization is 3 × 5 × 37; divisors are 1, 3, 5, 15, 37, 111, 185, 555.
• Electronics: the 555 timer IC (basics, astable/monostable modes, key formulas like f ≈ 1.44 / ((RA + 2RB)C)).
• Roman numerals: 555 = DLV.
• Other context (e.g., a year, a code, etc.).

Tell me your subject and level (undergrad, exam type) and which interpretation you want, and I’ll tailor a quick explanation or a few practice problems.