One twentieth is 5%, so the total cost is
f(c) = 1.05 c
The domain is [80, 180]
For Albert to afford the shoes
[tex]f(c) \le 105 + 42[/tex]
[tex]1.05 c \le 147[/tex]
[tex] c \le 147/1.05 = 140[/tex]
That's the maximum so he can afford shoes with preshipping prices in the domain
[80, 140]
