JSFiddle - React, Tailwind, and code Playground

CSS

span {
    display: block;
}

JavaScript

var dataArray = [{
    "Year": "Year 7",
        "test1": 28,
        "test2": 19,
        "test3": 22,
        "average": ''

}, {
    "Year": "Year 8",
        "test1": 15,
        "test2": 28,
        "test3": 22,
        "average": ''

}, {
    "Year": "Year 9",
        "test1": 25,
        "test2": 24,
        "test3": 22,
        "average": ''
}, {
    "Year": "Year 10",
        "test1": 26,
        "test2": 19,
        "test3": 22,
        "average": ''
}, {
    "Year": "Year 11",
        "test1": 48,
        "test2": 52,
        "test3": 22,
        "average": ''
}];


dataArray.forEach(function (e, i) {
    var average = Math.floor((e.test1 + e.test2 + e.test3) / 3);
    $('<span />', {
        text: e.test1 + ' + ' + e.test2 + ' + ' + e.test3 + ' / = ' + average
    }).appendTo('body');
    e.average = average;
});

console.log(dataArray);