555

Could you clarify what you want with "555"? Here are a few common possibilities:

• Math number: 555 = 3 × 5 × 37; divisible by 3 and 5; in binary 1000101011; in hex 0x22B; Roman numeral DLV.
• 555 timer IC (electronics): used in timing circuits; common formulas for astable mode: f ≈ 1.44 / ((RA + 2RB) × C) (units: RA, RB in ohms, C in farads).
• Another context (e.g., Thai slang, a different problem): please specify.

Tell me which topic you want and what kind of help (explanation, worked example, practice problems), and I’ll tailor it.