#container {
    width: 800px;
    height: 800px;
}

.square {
    width: 400px; 
    height: 400px;
    background-color: red;
    display: inline-block;
    float: left;
}

#squareBlue {
    background-color: blue;
}

#rectangleRed {
    width: 150px;
    height: 25px;
    background-color: red;
    display: inline-block;
}