Thành viên:Đức Anh/Nháp 56

    Tủ sách mở Wikibooks
    .group-layout {
        display: flex;
        flex-wrap: wrap;
    }
    
    .group-layout .group {
        flex: 0 0 calc(25% - 15px);
        margin-top: 20px;
    }
    
    .group-layout .group:nth-child(4n+1) {
        margin-right: 20px;
    }
    .group-layout .group:nth-child(4n+2) {
        margin-right: 20px;
    }
    .group-layout .group:nth-child(4n+3) {
        margin-right: 20px;
    }
    
    @media only screen and (max-width: 1650px) {
    
    .group-layout .group {
        flex: 0 0 calc(33.33% - 13.34px);
        margin-top: 20px;
    }
    .group-layout .group:nth-child(4n+1) {
        margin-right: 0px;
    }
    .group-layout .group:nth-child(4n+2) {
        margin-right: 0px;
    }
    .group-layout .group:nth-child(4n+3) {
        margin-right: 0px;
    }
    .group-layout .group:nth-child(3n+1) {
        margin-right: 20px;
    }
    .group-layout .group:nth-child(3n+2) {
        margin-right: 20px;
    }
    
    }
    
    @media only screen and (max-width: 1100px) {
    
       .group-layout .group {
            flex: 0 0 calc(50% - 10px);
        }
       .group-layout .group:nth-child(3n+1) {
            margin-right: 0px;
       }
       .group-layout .group:nth-child(3n+2) {
            margin-right: 0px;
       }
       .group-layout .group:nth-child(2n+1) {
            margin-right: 20px;
       }
    
    }
    
    @media only screen and (max-width: 550px) {
    
        .group-layout .group {
            flex: 0 0 100%;
        }
    
    }
    
    .article-text {
        padding-bottom: 6px;
        padding-left: 6px;
        position: absolute;
        bottom: 0;
        background-image: linear-gradient(rgba(0, 0, 0, 0), rgba(0, 0, 0, 1));
        width: calc(100% - 5px);
    }
    
    .p1 {padding: 0px 10px 0px 10px;}
    .p2 {padding-left: 10px;line-height:80%;}
    .toanhoc{
        background-image: url(https://upload.wikimedia.org/wikipedia/commons/8/89/Pure-mathematics-formul%C3%A6-blackboard.jpg);
        background-size: cover;
        width: 100%;
        padding-top: 100%; 
        position: relative; 
    }