.course{
    width: 100%;
    text-align: center;
    margin-bottom: 30px;
    }

.course_list{width:1200px;margin: 0 auto;}

.course_list ul li{
    float: left;
    display: inline-block;
    width: calc(100% / 3 - 20px);
    margin: 0 5px 10px 5px;
    border: solid 2px #dbbcf7;
    padding: 20px;
    min-height: 91px;
    font-size: 18px;
    line-height: 24px;
    text-align: center;
    color:#000;
    border-radius: 5px;
}
@media screen and (max-width:1200px){

.course_list{width:98%}
.course_list ul li{
    width: calc(100% / 2 - 20px);}
.course img{width:100%}
}

@media screen and (max-width:690px){
.course_list ul li{
    width: calc(100% / 1 - 20px);}
}