Thành viên:Đức Anh/Nháp 56

.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; 
}