#container {
    display: -webkit-flex;
    display: flex;
    -webkit-flex-flow: row wrap;
    flex-flow: row wrap;

    margin: 0;
    padding: 0;
    width: 100%;
}

#container div, header, nav, article, footer {
    box-sizing: border-box;
    min-height: 10px;
    min-width: 150px;
}

header, nav, article, footer {
    width: 100%;
}

header h1{
    font-size: 20px;
}

nav ul{
    list-style-type: none;
    margin: 0;
    padding: 0;
}

nav ul li{
    text-align: center;
}

footer{
    font-style: italic;
    font-size: small;
    text-align: center;
    padding: 5px;
}
nav a, footer a{
    color: lightgray;
}
@media (min-width: 600px) {

    nav ul{
        width:100%;
        height:auto;
    }

    nav ul li{
        float: left;
        padding: 15px;
    }


}

@media (min-width: 800px) {
    #container {
        width: 800px;
        margin-left: auto;
        margin-right: auto;
    }

    nav {
        width: 25%;
        min-height: 250px;
    }

    nav ul {
        width: 100%;
    }

    nav ul li{
        float: none;
        text-align: left;
    }

    article {
        width: 75%;
    }
}

header {
    color: #ddd;
    background-color: #003476;

}

header h1{
    text-align: center;

}

nav {
    color: #ddd;
    background-color: #0062d2;
}

article {
    color: #333;
    background-color: #b4d2f7;
    padding: 0px 20px;
}

footer {
    color: #ddd;
    background-color: #003476;
}

#num1, #num2, #ans{
    width: 50px;
}
#num1{
    margin-left: 12px;
}
#multiply{
    margin-left: 5px;
}

#errorMessage{
    display: none;
    width: 100%;
    text-align: center;
    padding: 10px;
    margin: 15px auto 5px;

    color: black;
    border: 5px solid rgb(190,50,38);
    font-weight: bold;
}