In a 12 storey house, 10 people enter a lift cabin. It is known that they will leave the lift in pre-decided groups of 2, 3 & 5 people at different storeys. The number of ways they can do so if the lift does not stop up to the second storey is

$$(2^{3n}-1)$$ is divisible by

$$\sum\limits_{n = 1}^m {n\,.\,n!} $$ is equal to

If ⁿC₃ = 220, then n = ?