math
posted by gaby .
if a gift is 42.40 including tax,what is the retail price for the gift if the sales tax is 6% ??

Set up a ratio. The cost of the gift including tax is 106% of the cost without tax.
42.40/106 = x/100
Solve for x.
(Note it does not matter whether you use 1.06 or 106 for the percentages in the ratio as long as you are consistent). 
1.06x = 42.4
x = 42.4 / 1.06
x = ? 
42.40 = X * 1.06
where X is the retail price
X = 42.40/1.06 = $40.00