#rectangulo {
    background-color: turquoise;
    height:400px;
    width:350px;
    border:5px solid black;
    left:50px;
    top:50px;
    position:absolute;
}

#pantalla {
    background-color: lightgrey;
    height:60px;
    width:245px;
    border:2px solid black;
    left:50px;
    top:50px;
    position:absolute;

}

#botones {
    width:245px;
    left:50px;
    top:150px;
    position:absolute;
    background-color: blue;
}


#botones-numeros {
    background-color: lightgreen;
    position: absolute;
    width: 150px;
}

#botones-operaciones {
    background-color: lightgreen;
    position: absolute;
    left: 150px;
}

button {
    border: none;
    color: black;
    padding: 10px 15px;
    text-align: center;
    display: inline-block;
    font-size: 16px;
    margin: 4px 2px;
    cursor: pointer;
}

h3 {
    margin: 4px 4px;
    text-align: right;
}

h5 {
    text-align: center;
    background-color: lightgreen;
}