Mean = 110
Sample size = 45
Standard deviation = 6
Z score for 90% is 1.645 ( from Z-table).
Lower Limit = Mean - Z(SD/√Sample size))
Lower limit = 110 - 1.645(6/√45)
Lower limit = 108.5
Upper Limit = Mean + Z(SD/√Sample size))
Lower limit = 110 +1.645(6/√45)
Lower limit = 111.5
The interval is (108.5, 111.5)
