I would like to add two strings together, so that if `7` and `8` are clicked, the function sets `outputbox`'s inner HTML to `78`. However, with this current function, it only shows the number most recently selected. How can I get it to show every number that's been selected?

```javascript
function selectNum(num)
{
    num = num + num // What goes here?
    document.getElementById("outputbox").innerHTML = num;
}

```

```html
<button onclick = "selectNum('7')">7</button>
<button onclick = "selectNum('8')">8</button>
<button onclick = "selectNum('9')">9</button>
<button onclick = "selectNum('4')">4</button>
<button onclick = "selectNum('5')">5</button>
<button onclick = "selectNum('6')">6</button>
<button onclick = "selectNum('1')">1</button>
<button onclick = "selectNum('2')">2</button>
<button onclick = "selectNum('3')">3</button>

```