You just have to find out the percent 7.15 is of 105. To do that, you set up a proportion with x as the percentage:
\(\frac{7.15}{105}=\frac{x}{100}\)
Then, cross multiply to get:
\(\frac{7.15}{105}=\frac{x}{100}\\715=105x\\105x/105=715/105\\x=6.810\)
So, the sales tax rate was 6.8%.
