I have two methods, but I am not sure if they are the methods you are looking for.
Let's say the price of the bill is $10, and you need to add a 20% tip.
You can do:
10 · 0.20 = 2
$2 is the tip, so 10 + 2 = 12
your total price is $12.
Another way is:
10 · 1.20 = 12
The "1" in 1.20 keeps the 10, and the ".20" adds the tip.
I think there are other ways to do it but I do not remember, sorry
