Answer:
See Explanation
Step-by-step explanation:
Given
In 1 hour:
Amy: 8 walls or 6 outlets
Bill: 5 walls or 5 outlets
Solving (a): Amy
(i) Opportunity cost of the outlets
This can be represented as:

Divide both sides by 2

This means that: for 3 outlets done, 4 walls are not done.
(i) Opportunity cost of the walls
This can be represented as:

Divide both sides by 2

This means that: for 4 walls done, 3 outlets are not done.
Solving (b): Bills
(i) Opportunity cost of the outlets
This can be represented as:

Divide both sides by 5

This means that: for 1 outlet done, 1 wall is not done.
(i) Opportunity cost of the walls
This can be represented as:

Divide both sides by 5

This means that: for 1 wall done, 1 outlet is not done.