Sum of all items including tax is

$$Totalcostof5items=\mathrm{\$}18.55$$

Let S be cost of items excluding tax:

$$S(1.06)=\mathrm{\$}18.55$$

$$S=\frac{\mathrm{\$}18.55}{1.06}=\mathrm{\$}17.50$$

$$Costof4items=6+2+4+3=\mathrm{\$}15$$

$$\therefore Bagofballoons=\mathrm{\$}17.50-\mathrm{\$}15=\mathrm{\$}2.50$$