Edit in JSFiddle

$(function () {
    var a, b, c, pm, timer,
    aMulti = 20,
        bMulti = 10,
        counter = 10,
        score = 0,
        minus = 50;
    makeQ = function () {
        a = Math.ceil(Math.random() * aMulti);
        b = Math.ceil(Math.random() * bMulti);
        if (Math.random() >= 0.5) {
            pm = "+";
            c = a + b;
            var ma = a,
                mb = b;
        } else {
            pm = "-";
            var ma = Math.max(a, b),
                mb = Math.min(a, b);
            c = ma - mb;
        }
        $(".retry").hide();
        $(".answer").show();
        $("#a").text(ma);
        $("#b").text(mb);
        $("#pm").text(pm);
        $("#score span").text(score);
        $("#counter").text("");
        for (var i = 1; i <= counter; i++) {
            $("#counter").append("♥️");
        }
        timer = setInterval(function () {
            $("#counter").text($("#counter").text().slice(2));
            if ($("#counter").text().length === 0) {
                $("#counter").text("😱");
                $("#c").text(c);
                $(".retry.timeup").show();
                clearInterval(timer);
            }
        }, 1000);
    };
    
    $(".retry").on("click", function () {
        $("#a, #b, #c").empty();
        if ($(this).hasClass("ok")) {
            score += ($("#counter").text().length / 2) * 10;
        } else {
            score -= minus;
            if (score < 0) {
                score = 0;
            }
        }
        makeQ();
    });

    $(".answer").click(function () {
        if (!$(".retry").is(":visible")) {
            $(".answer").hide();
            $("#c").text(c);
            clearInterval(timer);
            $(".retry.ok").show();
            $(".retry.ng").show();
        }
    });

    makeQ();
});

<p id="score">てんすう：<span></span>点</p>
<p class="q"><span id="a"></span>  <span id="pm"></span>  <span id="b"></span> = <span id="c"></span>
</p>
<p id="counter"></p>
<button class="answer">こたえがわかったら、こたえをいいながらおせ！</button>

<button class="retry ok">せいかいした！</button>
<button class="retry ng">まちがえた！</button>
<button class="retry timeup">じかんぎれ！</button>

body {
    font-family: sans-serif;
}
#score {
    text-align: right;
    font-size: 20px;
    margin: 10px;
    span {
        font-size: 40px;   
    }
}
p {
    text-align: center;
}
p.q,#counter {
    font-size: 50px;
    padding: 10px;
    margin: 10px;
    border: 2px solid #9af;
    border-radius: 4px;
    background-color: #acf;
}
#counter {
    font-size: 30px;
}
button {
    width: 40%;
    display: block;
    margin: 10px;
    padding: 10px;
    font-size: 20px;
}
button.answer {
    width: auto;
    margin: 10px auto;
}
.ok {
    float: left;
}
.ng {
    float: right;
}
.timeup {
    margin: 10px auto;
}