given U = {15,16,17,18,19,20, 21, 22, 23, 24, 25},

A = {15, 17, 21, 23, 25}

so A' will be elements of U that are not in A = U-A = {16,18,19,20,22,24}

B = {16, 17, 18, 22, 25}

so B' will be elements of U that are not in B = U-B = {15,19,20,21,23,24}

so A'UB' = {15,16,18,19,20,21,22,23,24}