Answer:
Domain[tex]=\{1,2,3,4,5,6,7,8,9,10,11,12,,13,14,15,16,17,18,19,20\}[/tex]
In other words Domain[tex]=\{x|x\ is\ an\ integer\ and\ 1\leq x\leq20\}[/tex]
Step-by-step explanation:
[tex]C(x)=80-4x[/tex]
Let after [tex]t[/tex] weeks she has left no money.
[tex]c(t)=0\\80-4t=0\\4t=80\\t=20[/tex]
That means she can have money for ice cream from the week 1 to week 20 and after that she will have no money.
So [tex]x[/tex] can be any number between [tex]1[/tex] and [tex]20[/tex]
Hence Domain[tex]=\{x|x\ is\ an\ integer\ and\ 1\leq x\leq20\}[/tex]