Question 184509
Note: the tick above the set name simply means the complement of the set. 


A′ = Everything in set U but NOT in set A = {18, 19, 20, 22, 24, 26, 28}


B' = Everything in set U but NOT in set B = {18, 19, 20, 21, 22, 23, 24, 25}



Now simply combine the two sets A' and B' (and remove any duplicate elements) to get



A′ U B′ = {18, 19, 20, 22, 24, 26, 28} U {18, 19, 20, 21, 22, 23, 24, 25} = {18, 19, 20, 21, 22, 23, 24, 25, 26, 28}



So the answer is


A′ U B′ = {18, 19, 20, 21, 22, 23, 24, 25, 26, 28}