Assume that

(n1,...,nq) ∼ Mult(N, p1,...,pq)

and let t be an integer less than q. Define y = n1 + ··· + nt and ˜p =

p1 + ··· + pt. Show that

(y, nt+1,...,nq) ∼ Mult(N, p, p ˜ t+1,...,pq)

so that E(y) = Np˜

and Var(y) = Np˜(1 − p˜).