.logo {
    width: 400px;
    margin:0px auto;
    display:inline-block;
    text-align: left;
    vertical-align: top;
    background:#cc0000;
}

.cajadelmenu {
    width: 500px;
    margin:0px auto;
    display: inline-block;
    text-align: right;
    vertical-align: top;
    background:#90c00d;
}

@media screen and (max-width:640px) { 
    .logo {
        width:99%;
        text-align: center;
        display: block;
    }
    .cajadelmenu {
        width:99%;
        display: block;
        text-align: center;
    }
}