@media (min-width: 641px) {
    #talk_body .txt_box {
        max-width: 700px;
        padding: 40px;
        margin: 0px 0px 30px 0px;
        box-sizing: border-box;
    }
}

@media (max-width: 640px) {
    #talk_body .txt_box {
        max-width: 480px;
        padding: 5px 5px 5px 5px;
        margin: calc(20 / 640 * 100vw) auto calc(60 / 640 * 100vw) auto;
        box-sizing: border-box;
    }
}