Factorials and nCr: working out values without a calculator, simplifying factorials with n, the numbers in Pascal’s triangle, counting choices, solving for n, symmetry and Pascal’s rule · nine levels, timed, full worked solutions
n! = n × (n − 1) × … × 2 × 1, and nCr = n!/(r!(n − r)!), the number of ways of choosing r things from n.