How is a 15% discount similar to a 15% decrease?
how is 8% sales tax like an 8% increase?

It is important to remember that a discount is the same as a decrease and a sales tax means an increase. If you remember this, you will easily know how to go about solving the mathematical questions.
A 15% discount is similar to a 15% decrease because a discount means a decrease in price and serves the purpose of a decrease.
An 8% sales tax is like an increase because it means that it is an 8% increase over the original price.