HTML -CSS help in calculator

I am making front end library project in plain HTML CSS and javascript.I have one problem here how to make the equal button longer than other buttons in the same row. Also in the code, I tried to do so why there is a gap in the previous buttons
My codepen:

This makes it show up better:

#equal{
  width:100px;height:65px;
 color:red;
font-size:2.5rem;
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