Float to the left with 45% width. I swear I'm copying exactly what he typed.

#gallery {
  margin: 0;
  padding: 0;
  list-style: none;

}
#gallery li: {
  float: left;
  width: 45%;
  margin: 2.5%;
  background-color: #f5f5f5;
  color: #bdc3c7;

}
#gallery li a p{ 
  margin: 0;
  padding: 5%;
    font-size: 0.75ems;
    color: #bdc3c7;
}

css/main.css
a {
  text-decoration: none;
}
img {
  max-width: 100%;

}
#wrapper {
  max-width: 940px;
  margin: 0 auto;
}

#logo {
  text-align: center;
  margin: 0;
}

h1, h2 {
  color: #fff;
}

nav a {
  color: #fff;
}

nav a:hover {
  color: #32673f;
}

h1 {
  font-family: ‘Changa One’, sans-serif;
  font-size: 1.75em;
  font-weight: normal;
}

footer {
  font-size: 0.75;
    text-align:center;
    clear: both;
    padding-top: 50px;
    color: #ccc;

}
#gallery {
  margin: 0;
  padding: 0;
  list-style: none;

}
#gallery li: {
  float: left;
  width: 45%;
  margin: 2.5%;
  background-color: #f5f5f5;
  color: #bdc3c7;

}
#gallery li a p{ 
  margin: 0;
  padding: 5%;
    font-size: 0.75ems;
    color: #bdc3c7;
}