The answer is $30.06.
John added 7% of tax to the price before tax which is 100%. So, the price John paid is 107%. If $32.16 is 107%, how many $ is 100% (the price before tax):
$32.16 : 107% = x : 100%
After crossing the products:
x · 107% = $32.16 · 100%
⇒ x = $32.16 · 100% ÷ 107% = $30.06
Therefore, the cost of groceries before the sales tax is $30.06.
