555

Could you clarify what you mean by “555”? Here are the two most common possibilities:

• 555 timer IC (electronics): quick refresher on monostable and astable modes, key formulas, and a sample calculation.
• The number 555 in math: prime factorization, divisors, and a couple of quick property checks.

If you’d like, I can give you a short cram sheet for one of these right away or generate a few practice problems. Which would you prefer?