JSFiddle - React, Tailwind, and code Playground

HTML

<label class='next action-button'>
    <input id="calc-button" type="button" name="tank" value="Yes" />
    Yes
</label>
<br/>
<p>standard and premium system specs</p>
<br/>
<div id="totalPrice"></div>
<br/>
<div id="premPrice"></div>

JavaScript

function calculateTotal() {

    var boilerPrice = getBoilerSizePrice() + getBedroomSizePrice() + getBathroomSizePrice() + getTankSizePrice();

    //display the result
    var divobj = document.getElementById('totalPrice');
    divobj.style.display = 'block';
    divobj.innerHTML = "Your New Boiler Install Price is £" + boilerPrice;

    var divobj = document.getElementById('premPrice');
    divobj.style.display = 'block';
    divobj.innerHTML = "Premium price £" + ((boilerPrice / 100) * 120);

}

function getBoilerSizePrice() {
    return 42;
}

function getBedroomSizePrice() {
    return 32;
}

function getBathroomSizePrice() {
    return 23;
}

function getTankSizePrice() {
    return 24;
}

document.getElementById("calc-button").addEventListener("click", calculateTotal);