The total was 18.55
The tax was 6%
cost before tax was x
x * ( 1.06) = 18.55
Divide each side by 1.06
x = 18.55/1.06
x =17.5
This is the price of all the items before tax
The 4 items on the receipt are 2,6,3,4 = 15
17.5 - 15 = 2.5
The missing item must be 2.50