$20.1
According to the information given:
The amount of money Zoe had on Monday is x dollars.
If her sister took $4.50 from Zoe on Tuesday, the balance on Tuesday will be:
[tex]x-4.50[/tex]If Zoey's mother gave her $20 on Wednesday, her balance on Wednesday will be expressed as:
[tex]\begin{gathered} x-4.50+20 \\ x+15.50 \end{gathered}[/tex]If Zoey has $17.80 left after she spent half of her money on Thursday, then the resulting expression will be:
[tex]\begin{gathered} (x+15.50)-\frac{1}{2}(x+15.50)=17.80 \\ \frac{1}{2}(x+15.50)=17.80 \end{gathered}[/tex]Cross multiply and get the value of x:
[tex]\begin{gathered} x+15.50=2(17.80) \\ x+15.50=35.6 \\ x=35.6-15.50 \\ x=\$20.1 \end{gathered}[/tex]This shows that Zoey has $20.1 on Monday
