Answer
Find out the cost of the dinner .
To proof
Let us assume that the cost of the dinner be x.
As given
The Silver Town people went to a fancy restaurant after the big event.
Grammy gave the server a $15 tip.
If the tip was 20% of the cost of dinner.
20% is written in the decimal form

= 0.2
Than the equation becomes
xĂ— 0.2 = 15

x =$75
The total cost of the dinner is $75 .
option (C) is correct .
Hence proved