11.25 kg of sand.
The coefficient of static friction tells you what fraction of the normal force is needed to start the object moving. Since the block is on a horizontal surface, the normal force is the same as the mass of the block. Therefore
M = 28 * 0.450 = 12.60
So once the mass hanging off the cord reaches 12.60 kg, the block will start to move. And since the bucket masses 1.35 kg, you need to subtract the mass of the bucket from the total mass needed, giving
M = 12.60 - 1.35 = 11.25
