how many two - element subsets of { 6,7 , 8,9 } are there that do not contain the pair of elements 8 and 9 ?
required subsets are = { 6,7 } , { 6,8 } , { 6,9 } , { 7,8 } , { 7,9 } = 5 answer : a
subtract(9, const_4)