163.Handshakes Among Four Couples
★★★combinatoricsJane Street
Eight people, forming four couples, all shake hands with each other exactly once — except that nobody shakes hands with their own partner. How many handshakes occur?
Sign in to submit an answer and track it toward your stats.
Sign inHint
Count all pairs first, then remove the ones that do not happen.
Want timed drills, mental math, and a market-making game too? Try the full practice suite →