How can I work out form submission calculations using JavaScript? [closed]

I have built an order form with lots of different products on it for a butchers. Each product has the following:

<div class="col text center">Shin of Beef <br>€9.50 per kilo</label>
    <br>
    <select id="old english" name="oldenglish">
        <option value="0">---------</option>
        <option value="1">1 Kilo</option>
        <option value="2">2 Kilos</option>
        <option value="3">3 Kilos</option>
        <option value="4">4 Kilos</option>
        <option value="5">5 Kilo</option>
        <option value="6">6 Kilos</option>
        <option value="7">7 Kilos</option>
        <option value="8">8 Kilos</option>
        <option value="9">9 Kilos</option>
        <option value="10">10 Kilos</option>
        <option value="11">11 Kilos</option>
        <option value="12">12 Kilos/option>
        <option value="13">13 Kilos</option>
        <option value="14">14 Kilos</option>
        <option value="15">15 Kilos</option>
        <option value="16">16 kilos</option>
        <option value="17">17 Kilos</option>
        <option value="18">18 Kilos</option>
        <option value="19">19 Kilos</option>
        <option value="20">20 Kilos</option>
    </select>

    </select>
    <br><label for="weight">Enter Weight</label>
    <input type="number">

I need to know how I can use JavaScript to select an option and show a price so say 2 packs is €9.50 x2 and it will calculate the prices once 2 packs are selected.