1. The problem statement, all variables and given/known data 2. Relevant equations Concept of couple? 3. The attempt at a solution First, I calculated the couple about disc, though I'm not sure if I can use this approach. Since 900 N is pulling the string of pulley downwards, I take the couple about disc as 900 x 0.4, which gives me 360 Nm. To balance this couple, the couple by F must be equal to 360. So 1.2 x F = 360 which gives me 300 N as the answer. But I'm wrong, since the answer is B. Where did I go wrong?