--Answer--
1. $85
2. $15
--Explanation--
1. 15% off of 100 just means moving the decimal in 15 over by 2 to the left.
From there, multiply 100 x 0.15.
Then you get 15, so that's how much tax you are paying. Meaning Subtract 15 from 100 to get the price it originally was before tax. 100 - 15 = 85.
2. $15 dollars tax from moving decimal 2 to the left of 15%.