Let s=(na1, n2a2, .. nk ak, and n=n+n2+.+=s, then the number n of r-combinations of s equals (1)0 r>n r-n 3)N=C(k+r-1, r) n; 2r for each i=1, 2,..., n (4 )If r<n, and there is, in general, no simple formula for the number of r-combinations of s. nonetheless a solution can be obtained by the inclusion-exclusion principle and technique of generating functions, and we discuss these in 4.5 and 4.6▪ Let S={n1 •a1 ,n2 •a2 ,…,nk •ak }, and n=n1+n2+…+nk=|S|， then the number N of r-combinations of S equals ▪ (1) 0 r>n ▪ (2) 1 r=n ▪ (3) N=C(k+r-1,r) ni r for each i=1,2,…,n. ▪ (4) If r<n, and there is, in general, no simple formula for the number of r-combinations of S. Nonetheless a solution can be obtained by the inclusion-exclusion principle and technique of generating functions, and we discuss these in 4.5 and 4.6