to find how much elwyn should leave for the tip, you multiply 75 by 0.20 to find the tip hes gonna give. (you turn the 20% tip into 0.20)
75 x 0.2= $15 <— this number means that elwyn is gonna give a $15 tip.
to find the total bill for the dinner with the tip too, you need to add $15 to his original bill cost, $75.
75 + 15= 90
Therefore, Elwyns total bill including the dinner and tip is $90