If ab=40 then 20/b= to what

Is there a picture of the diagram?

F=31 because you subtract 39-8

The answer is c, i, iii, and iv. this is because farming and mining deal with harvesting raw materials, construction work falls under manufacturing, and piloting a plane is a service. ii is wrong because making bread is using  raw materials to produce goods, which would fall under the secondary sector, not primary.