STEP 1:
find the decimal version of the tax rate
x= tax rate
cost of item * x= tax total
$340x= $10.20
divide both sides by 340
x= 10.20/340
x= 0.03
STEP 2:
find the percent version of the tax rate; multiply decimal version by 100 (or simply move the decimal two places to the right)
= 0.03 * 100
= 3%
CHECK:
$340 * 3%= $10.20
340 * 0.03= 10.20
10.20= 10.20
ANSWER: The tax rate is 3%.
Hope this helps! :)