555

Could you clarify what you mean by "555"?

• If you mean the 555 timer IC (electronics), I can give a quick study guide: pinout, the three modes (astable, monostable, bistable), and common formulas.
• If you mean something else (math problem, the number 555 in a different context, etc.), tell me the topic and I’ll tailor the help.

If you want the electronics quick refresher now, I can provide a concise cheat-sheet.