The set a consists of all 10-combinations of d in which a occurs at least 4 time If we take any one of these 10-combinations in a and remove 4 a's. we are left with a 6 combination of d Conversely, if we take a 6-combination of D and add 4 as to it, we get a 10-combination of D in which a occurs at least 4 times Thus the number of 10-combinations in A1 equals the number of 6-combinations of D Hence,|A=C(3+6-1,6)=((8,6=C(8,2),▪ The set A1 consists of all 10-combinations of D in which a occurs at least 4 time. ▪ If we take any one of these 10-combinations in A1 and remove 4 a’s, we are left with a 6- combination of D. ▪ Conversely, if we take a 6-combination of D and add 4 a’s to it, we get a 10-combination of D in which a occurs at least 4 times. ▪ Thus the number of 10-combinations in A1 equals the number of 6-combinations of D. Hence, |A1 |=C(3+6-1,6)=C(8,6)=C(8,2)