Володимир,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<span class="firstNumber"></span>
<span class="sign"></span>
<span class="secondNumber"></span>
<button id="plus">START</button>
<script>
var firstNumb = document.querySelector(".firstNumber");
var secondNumb = document.querySelector(".secondNumber");
var signn = document.querySelector(".sign");
var numb = 100;
var signArray = ["+","-","*","/"];
function randomNumber(numb){
return Math.floor(Math.random()*numb);
}
function rand(){
var firstNumber = randomNumber(numb);
var secondNumber = randomNumber(numb);
var randomSign = signArray[randomNumber(signArray.length)];
if(randomSign === "/" && !secondNumber) secondNumber = randomNumber(numb) + 1;
putInBox(firstNumber,secondNumber,randomSign);
}
function putInBox(firstNumber,secondNumber,randomSign){
firstNumb.innerHTML = firstNumber;
secondNumb.innerHTML = secondNumber;
signn.innerHTML = randomSign;
}
document.getElementById("plus").addEventListener("click",rand)
</script>
</body>
</html>