28/28 100% гляньте что не так

html,
body {
margin: 0;
padding: 0;
}

body {
width: 380px;
min-height: 280px;
padding: 10px;
font-family: “Arial”, sans-serif;
font-size: 14px;
line-height: 1.4;
background: #ecf0f1;
}
.main-menu {
width: 200px;
margin: 0;
padding: 0;
list-style: none;
background: #2980b9;
}

.main-menu a {
display: block;
padding: 10px 15px;
text-decoration: none;
color: #ffffff;
border-bottom: 2px solid #2c3e50;

}
.main-menu .sub-menu {
position: absolute;
top: 0px;
left: 180px;
width: 200px;
margin: 0;
padding: 0;
list-style: none;
background-color: #1abc9c;
z-index: 100;

}
.main-menu .sub-menu a{
border-bottom: 2px solid #2ecc71;
}
.main-menu > li {
position: relative;
}
.main-menu.opened.sub-menu {
display: block;

}
.main-menu > .opened > a {
background-color: #1abc9c;

}
.main-menu.opened {
border-top: 2px solid #2c3e50;
}