
For

the derivative is negative.
For

the derivative is positive.
Therefore at

there's a minimum.
Answer: C) 0
The lines are parallel. They never cross. To have a solution, the lines need to intersect. Each intersection is a solution. That's why there are no solutions here. The system is inconsistent.
Note how the slopes are both 1/2 while the y intercepts are different. Parallel lines have equal slopes but different y intercepts.
Answer: 20x < 100
subtract 20 from both sides
20 - 20 = 0
100 - 20 = 80
x < 80