1) Whenever we are dealing with calculating how much are you going to give as a TIP,
For example:
Let's say you want to give a 20% tip over a $35 bill
Then multiply :
35 x ( 1 + 0.2) =
35 x (1.2)=
42 That's how much in total you're going to pay ($35 + $7 tip)
2) In case, you want to calculate the discounted price then proceed like this:
Ex.
Suppose the manager has given you a 20% off on your bill
35 x ( 1 -0.2)
35 x ( 0.8)
$28
Then this is going to be your discounted price (the tip would be included over).