JSFiddle - React, Tailwind, and code Playground

HTML

<button id="start">Start</button>
<div>
    <span id="formula"></span>
    <input type="text" id="answer"/>
</div>

CSS

input[type='text'] { width: 3em; }

JavaScript

var button=document.getElementById('start'); // Кнопка
var formula=document.getElementById('formula'); // Формула
var answer=document.getElementById('answer'); // Тестовое поле

var a,b,time;
button.onclick = function () {
    a = Math.round(Math.random()*99);
    b = Math.round(Math.random()*(99-a));
    formula.textContent=a + ' + ' + b + ' = ' ;
    time=Date.now();
};
answer.onkeyup=function(){
    if (this.value-0 === a+b) {
        alert('Время ' + (Date.now()-time)/1000 + ' сек' );
    }
}