A. 5000/2.5=2000 and 5000/3.25=1538.5. If he only makes $2.50 items he can't exceed 2000, and if he only makes $3.25 items he can't exceed 1538. So the inequality is 2.5A+3.25B<5000 where A represents the number of $2.50 items and B that of $3.25 items.
B. To graph the inequality, we write one item in terms of the other: 3.25B=5000-2.5A. If B is the y axis and A the x axis we get a backward sloping straight line. The y intercept is 5000/3.25=1538.5 and the x intercept is 5000/2.5=2000. Choose a suitable scaling and join the intercepts. The graph only has a positive region so the origin of the axes can be placed very much on the left and towards the bottom of the page. The shaded area is the region bounded by the line and axes.
C. Choose the three points P, Q and R as directed.
P is within the shaded area and represents (A,B) in budget; Q makes use of the whole investment; R is outside budget (exceeds the investment). Although the graph is a straight line, in reality it is a sequence of points because A and B cannot contain fractions. If the graph has been drawn reasonably accurately it should be possible to read off values of A and B. The values A and B must be integers, so they need to be rounded down to the nearest integer.
To work out the cost calculate C=2.5A+3.25B. The quantity Q=A+B. The average unit cost=C/Q.