flexbox.css

.c-posts {
  display: flex;
  flex-wrap: wrap;
  align-items: top;
  padding: 4em;
}
.c-posts__item {
    flex-grow: 1;
    padding-bottom: 5em;
    
    &:first-child {
      flex-grow: 2;
    }
}

@media all and (min-width:600px) {
  .c-posts {
    &__item {
      flex-basis: 50%;
      padding-right: 5%;
      
      &:first-child {
        flex-basis: 100%;
      }
    }
  }
}

@media all and (min-width:1000px) {
  .c-posts {
    &__item {
      flex-basis: 33%;
      
      &:first-child {
        flex-basis: 66%;
      }
    }
  }
}

@media all and (min-width:1400px) {
  .c-posts {
    &__item {
      flex-basis: 25%;
      
      &:first-child {
        flex-basis: 50%;
      }
    }
  }
}

/* Other Non-Flexbox Related Styles */
.c-header,
.c-footer {
  background: #EDD382;
  text-align: center;
  padding: 2em;
  font-family: 'Source Sans Pro', sans-serif;
  font-size: .8em;
  font-weight: 300;
  line-height: 1.6em;
  
  h1, h2, p {
    max-width: 40em;
    margin: 0 auto;
    
    &:not(:last-child) {
      margin-bottom: 1em;
    }
  }
  
  h1 {
    text-transform: uppercase;
    font-weight: 900;
  }
  
  a {
    color: #000;
  }
}

.c-footer {
  p {
    font-weight: 300;
  }
}

* {
  box-sizing: border-box;
}

body {
  font-family: 'Georgia', Times, serif;
  line-height: 1.6em;
  padding: 0;
  margin: 0;
}

h1 {
  font-size: calc(130% + 1vw);
  font-weight: normal;
}

h2 {
  font-size: 1.5em;
  font-weight: normal;
  margin-bottom: 0;
  margin-top: 0;
}

a {
  color: #0F5257;
}

.c-btn {
  color: #000;
  display: inline-block;
  border-bottom: 4px solid #000;
  text-decoration: none;
  text-transform: uppercase;
  font-family: 'Source Sans Pro', sans-serif;
  font-weight: 900;
  letter-spacing: .1em;
  padding: .3em 0;
  position: relative;
  transition: .2s all;
  
  &:before {
    content: '';
    position: absolute;
    bottom: 0;
    width: 100%;
    background: #FF521B;
    height: 0%;
    transition: .2s all;
    left: 0;
    z-index: -10;
  }
  
  &:hover {
    padding-left: .75em;
    padding-right: .75em;
    color: #fff;
    border-color: #FF521B;
    
    &:before {
      height: 100%;
    }
  }
}