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You have calculated correctly the power dissipated by each resistor in each case.
However, in engineering you have to leave a safety margin when selecting components. Ideally, the higher the safety margin the better, but practically, there is a cost (and size) implication so you have to strike a balance between the two. After calculating the power dissipation (say P) like you did, different folks use different safety factors to obtain the minimum power rating for the resistor. There are factors like 1.25*P; 1.3333*P; 1.5*P and so on. Here, 1.25*P gives the least safety margin and 1.5*P gives the most. I go for 1.5*P.
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