Concept:A total is possible only if it can be formed by adding some of the weights 20,40,60,90, using each box at most once.Explanation:Check the given options using subset sums. 120=20+40+60, so 120 is possible. 150=60+90, so 150 is possible. 170=20+60+90, so 170 is possible. 160 cannot be formed by any combination of 20,40,60,90. Hence, 160 kg cannot be the total weight.Answer:160 kg cannot be the total weight.