For question 4v), why, if they are taking moments along the line 0z, why is it 30Q=(60 x 17)??? Isn't the perpendicular distance from 0z to the force Q 40???? The mark shceme is provided if you keep scrolling down. All you really need to know is that the x co-ordinate of the centre of mass is 17 and the weight of 60N acts through the centre of mass. (hence where the 60x17) comes from.
I havent done the question.. but from what I can see isnt the distance 30 if you are looking at the top of the bracket, which is where the force is acting? look at Fig 4.2 again..