Level 7: Combinatorics & counting
Permutations, combinations, stars & bars, inclusion–exclusion, Catalan numbers, and pigeonhole — the counting machinery behind every probability.
12 problems
First 2 free · rest Premium
- 1→
Counting Handshakes
●○○○○Warm-up~2 min
- 2→
Stars and Bars
●●●○○Medium~5 min
- 3Premium
Monotone Lattice Paths
●●○○○Easy~4 min
- 4Premium
Inclusion–Exclusion
●●●○○Medium~5 min
- 5Premium
Arrangements of MISSISSIPPI
●●○○○Easy~4 min
- 6Premium
The Hockey-Stick Identity
●●●○○Medium~5 min
- 7Premium
Balanced Parentheses
●●●○○Medium~6 min
- 8Premium
Friends and Strangers
●●●●○Hard~7 min
- 9Premium
Subsets With No Two Adjacent
●●●○○Medium~5 min
- 10Premium
Splitting Into Groups
●●○○○Easy~4 min
- 11Premium
Counting Necklaces
●●●●○Hard~7 min
- 12Premium
A Committee With a Required Member Type
●●○○○Easy~3 min