How to add new query ?/

Tell us what’s happening:
Describe your issue in detail here.

  **Your code so far**
/* file: index.html */
<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Photo Gallery</title>
  <link rel="stylesheet" href="./styles.css">
</head>
<body>
  <div class="header">
    <h1>CSS FLEXBOX PHOTO GALLERY</h1>
  </div>
  <div id="gallery">
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/1.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/2.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/3.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/4.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/5.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/6.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/7.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/8.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/9.jpg"/>
    <img src="https://cdn.freecodecamp.org/curriculum/css-photo-gallery/10.jpg"/>
  </div>
</body>
</html>
/* file: styles.css */
* {
box-sizing: border-box;
}

body {
margin: 0;
font-family: Arial;
background: #EBE7E7;
}

.header {
text-align: center;
padding: 32px;
background: #E0DDDD;
}

#gallery {
display: flex;
flex-direction: row;
flex-wrap: wrap;
justify-content: center;
align-items: center;
padding: 0 4px;
}

#gallery img {
width: 25%;
height: 300px;
object-fit: cover;
margin-top: 8px;
padding: 0 4px;
border-radius: 10px;
}

#gallery img{
width: 50%;
@media(max-width: 800px;)
}
  **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/102.0.0.0 Safari/537.36

Challenge: Step 20

Link to the challenge:

Not sure when media queries were taught in the curriculum but your syntax is not correct.

The selector goes inside the media query and the media query condition does not use a semicolon.

Example code:

@media (max-width: 400px) {
  .card {
    background: red;
  }
}