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